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List All Datasheets

In each of the data sheets, a diagram is used to show the type of unit and the kind of signals that come into it and go out from it.

Input Sensors – detect changes outside the electronic system

	555 Astable Turns something on, and then off, and keeps repeating this.
	Debounced Switch Produces a switch that produces a sharp change from low to high, suitable for use with a counter.
	Hall Effect Sensor The Hall effect sensor produces an output sighnal that changes when a magnet is near.
	Infrared photo module This unit is used for remote control using infrared over a distance of up to about 10m. Depending on the circuit details, it can provide simple on-off operation, or it can receive data from a hand-held remote control or a PIC.
	Keypad A keypad is used for data entry. It needs to be used with a PIC.
	Light Sensor The light sensor is used to detect changes in the amount of light in its surroundings.
	Linear temperature sensor Linear temperature sensors are used to monitor temperature accurately. They are particularly useful with PICs which have an analogue to digital converter.
	Moisture sensor The moisture sensor is used to detect changes in the wetness of its surroundings.
	Movement sensor The movement sensor is a switch. It is 'on' (closed) if the sensor is not moved. With the circuit arrangement suggested this gives a low output signal. If the sensor is moved (accelerated) the switch opens and the signal goes high.
	Optoswitch There are two types of optoswitch - the slotted optoswitch and the reflective optoswitch. Both are used to detect nearby objects. The slotted optoswitch detects when an object is in the slot. The reflective optoswitch detects when a reflective object is near the optoswitch.
	Photodetector Photodectors are semiconductor devices which respond to light. They can replace light dependent resistors and have the advantage of lower pollution and smaller size.
	Pulse Unit Turns something on, and then off, and keeps repeating this.
	Quantum Tunnelling Composite QTC comes in the form of sheet, 'pills' and cable. QTC sheet can be used to make very low cost touch switches that can be placed on the case of products. QTC 'pills' can be used to produce low cost force/pressure sensors whose resistance varies with the applied force.
	Reed Switch / Proximity Switch The reed switch is used to detect the presence of a magnet.
	Rotation Sensor / Voltage Unit The rotation sensor uses a potentiometer to sense rotation. It can also be used to produce a variable voltage.
	Sound Sensor The sound sensor provides an output signal voltage that responds to sound detected by a microphone. It will only respond to loud sounds.
	Switch Unit The switch subsystem provides a switch that closes when pressed. The output signal from the switch subsystem goes high when the switch is pressed.
	Temperature Sensor (Thermistor) The temperature sensor is used to detect changes in the temperature of its surroundings.
	Tilt Switch The tilt switch is ued to detect movement.

Process Units – change the electronic signals

	555 Monostable The 555 monostable subsystem provides an output signal that is triggered high for a period of time before returning to low.
	AND Gate Makes something happen when both inputs are activated.
	CMOS Integrated Circuits CMOS integrated circuits are widely used as Process units.
	Comparator The comparator provides a large change in signal when the input signal only changes slightly and converts an analogue signal into a digital signal. It 'compares' the voltage input signal and the voltage from a potentiometer.
	Counter The counter subsystem counts the number of signal pulses connected to its clock input.
	Delay The delay subsystem produces a delay after the input signal goes high.
	Difference Amplifier (or Subtractor) The difference amplifier is used with two analogue input signals. It gives an analogue output signal which is proportional to the difference between the two input voltages.
	Exclusive OR Gate (XOR/EOR) Makes something happen when either, but not both, inputs are activated.
	Inverter (NOT Gate) The inverter subsystem, also know as a NOT gate, provides an output signal which is opposite to the input signal. When the input signal is high, the output signal is low, and vice versa.
	NAND Gate Turns something off when both inputs are activated.
	Negative Latch The negative latch produces an output signal that goes high and remains high when the input signal has been low. It is useful for turning something on until a second signal switches it off.
	Non-Inverting Amplifier The non-inverting amplifier subsystem is used to amplify an analogue input signal.
	NOR Gate Turns something off when either input is activated.
	OR Gate Turns something on when either input is activated.
	PIC Microcontrollers PIC microcontrollers are general purpose programmable process units.
	Positive latch The positive latch produces an output signal that goes high and remains high when the input signal has been high. It is useful for turning something on until a second signal switches it off.
	Retriggerable Monostable The retriggerable monostable subsystem produces a delay after the input signal goes high.
	Schmitt Inverter The Schmitt inverter subsystem, also known as a Schmitt NOT gate, provides an output signal which is opposite to the input signal. The Schmitt inverter is ideal for converting analogue signals into digital signals.
	Summing Amplifier The summing amplifier subsystem is used to add two analogue input signals together.

Driver Units – boost the power from process units

	Darlington Driver The Darlington driver subsystem is an electronic switch that provides an output signal powerful enough to drive output subsystems requiring high current.
	L293D Driver The L293D driver subsystem is particularly useful for use with d.c. motors because it can control two motors and can drive them forwards and backwards.
	MOSFET (Transducer Driver) The MOSFET driver subsystem is an electronic switch that provides an output signal powerful enough to drive output subsystems requiring very high current.
	Relay The relay subsystem is an electrically-operated switch. It requires a separate electrical supply to provide power to an output device. It is often used for reversing motors.
	Thyristor The transistor driver subsytem is an electronic switch that provides an output signal powerful enough to drive output subsytems requiring medium current.
	Transistor A thyristor is used to drive a load. It is switched on by applying a positive voltage to its input pin (the 'gate').

Output Devices – make something happen outside the electronic system

	7 Segment Display The 7-segment display is used to display numbers.
	Bar Graph Display A bargraph display is usually used to give a visual indication of an analogue voltage signal.
	Bulb (or Lamp) The bulb subsystem converts the input signal into light.
	Buzzer (and Piezo sounder) The buzzer subsystem produces an audible tone when powered.
	LED The LED (light-emitting diode) subsystem converts the input signal into light.
	Liquid Crystal Display (LCD) Liquid crystal displays are very useful for displaying both text and numbers. They display information sent from a PIC.
	Loudspeaker Loudspeakers are used to produce sounds.
	Motor The motor subsytem provides rotational motion when powered.
	Piezo Transducer Piezo transducers are used to produce sounds.
	Servo Motor Servo motors turn through a precise angle. They are controlled by a series of pulses. The width of the pulses to the servo motor controls the angle through which it turns.
	Solenoid The solenoid subsystem provides linear motion.
	Sound and Music Units are available that will play a variety of tunes and interesting sounds and that record and replay sounds.
	Stepper Motor A stepper motor turns through precise steps. So it is useful for moving things through an exact angle or distance.

We are grateful to New Wave Concepts for permission to adapt these data sheets based on their own data sheets for Control Studio software.

Other Subsystems and Components

Alternative Power Sources

	Battery Batteries are used to provide electrical power to portable electronic systems.
	Capacitors - Component Capacitors store electrical charge. They are often used for reducing 'noise' on a voltage or for controlling timing operations.
	Resistor - Component Resistors resist the flow of electricity. They are used in a wide variety of electronic subsystems to regulate current and control voltage.
	Voltage Regulator Voltage regulators are used to produce a stable power supply voltage (+Vs) from a higher (varying) d.c voltage. They can also be used to limit the current supplied.

Notes on Combining Subsystems from data sheets

• Combining Subsystems
• Circuit Wizard
• Livewire and PCB Wizard
• PCB Wizard
• Crocodile Technology and real-PCB
• Other PCB Design Software

555 Astable - Input Unit

What does it do?

Turns something on, and then off, and keeps repeating this.

How does it operate?

The 555 Astable provides an output signal that is high, then low, then high, low...
The time between the signal going from high to low and going from high to low again can be controlled by the values of two resistors.

A mark : space ratio is the ratio of the ‘mark’ time to the ‘space’ time.

From these two calculations we can obtain the frequency (the number of oscillations per second) of the pulse generated by the 555 astable:

So, if C = 10µF, R1 = 100k and R2 = 150k, the frequency is 0.36 Hz (which is 1/2.7s).

The 555 timer IC works with a d.c. power supply with a voltage between 4.5V and 16V. The 555 timer is able to provide an output current of 100mA and can therefore drive low and medium current output devices directly.

Capacitor types
Ceramic disc capacitors should not be used for the timing capacitor C. They are not sufficiently stable in capacitance to operate properly for timing. Suitable capacitor types are: silver mica, mylar, polycarbonate, polystyrene, tantalum, or similar types.

Possible applications

• Flashing a LED or bulb
• Sounding a buzzer on and off

Making

Pins of 555 timer IC

How part of the PCV might look

Use a Dual In Line (DIL) socket for the IC. Before inserting the IC, connect the power supply and use a voltmeter to check that the voltage on pin 1 is low (0V) and the voltage on pins 4 and 8 is high (the supply voltage).

Connect the negative lead of the capacitor to 0V. Insert the IC the right way round.

Testing

Make sure that the signal going out (on the green PCB track) changes from high to low and that the time period is correct.

Fault finding

If there is a fault, check that:

• The voltage on pin 1 is low (0V)
• The voltage on pins 4 and 8 is high (the supply voltage)
• The capacitor and IC are the right way round

If there is a fault, check the tracks and solder joints.

Alternatives

• Pulse Unit – cheaper but less accurate time period and fixed mark : space ratio.
• PICs – more flexible but more expensive.

Web links

• Inputs and outputs
• Manufacturer's datasheet for 555 timer IC

Return to list of datasheets

Debounced Switch - Input Sensor

What does it do?

Provides a switch that produces a sharp change from low to high, suitable for use with a counter.

How does it operate?

Possible Actions

• Triggering a counter
• Triggering a PIC being used to do something as soon as a switch is pressed (sometimes an ordinary switch can be used with a short delay in the PIC software).

Making

Pins of 40106B Schmitt inverter

How part of the PCB might look

The PCB shows the basic circuit. Several gates in the IC are not used in this simple design; they can be applied in other subsystems. Any unused input pins should be connected to 0V or Vs, to prevent damage by static electricity.

Use a Dual In Line (DIL) socket for the IC. Before inserting the IC, connect the power supply and use a voltmeter to check that the voltage on pin 7 is low (0V) and the voltage on pin 14 is high (the supply voltage).

Connect the negative lead of the capacitor to 0V. Insert the IC the right way round.

Testing

Make sure that the signal going out (on the green PCB track) changes from low to high when the switch is pressed.

Fault finding

If there is a fault, check that:

• The voltage on pin 7 is low (0V)
• The voltage on pin 14 is high (the supply voltage)

If there is a fault, check the tracks and solder joints.

Alternatives

• Push Switch – cheaper, but the switch bounces, which can give a problem with counters.

Web links

• Inputs and outputs
• Manufacturer’s data sheet for 40106B
• Data sheet on CMOS ICs

Hall Effect Sensor - Input Unit

What does it do?

The Hall effect sensor produces an output signal that changes when a magnet is near.

How does it operate?

If a south magnetic pole (or face) of a magnet is brought next to the front face of the sensor the output signal voltage increases. A north magnetic pole (or face) decreases the output signal voltage.

The rate of change of the output signal voltage with magnetic field is typically 2.5 V per tesla

If the signal from the Hall effect sensor is to be used with a digital system then the analogue signal from the sensor needs to be fed to a comparator.

Possible applications

• Sensing the closeness of a magnet, or any object wit an attached magnet
• Measuring the strength of a magnetic field

Making

Pins of Hall effect sensor

How part of the PCB might look

Testing

Make sure that the signal going out (on the green PCB track) changes from half of the value of +Vs (the supply voltage) when a magnet is brought near.

Fault finding

If there is a fault, check that:

• The voltage on the centre pin is low (0V)
• The voltage on the left-hand pin in the above pin diagram is high (the supply voltage)

If there is a fault, check the tracks and solder joints.

Alternatives

• A reed switch/proximity switch can also detect the presence of a magnet. The Hall effect sensor has the advantages of being more robust and not producing switch bounce. A Hall effect sensor is more expensive than a reed switch.
• A microswitch or a light sensor can be used to detect if an object is near.
• Optoswitches are useful for detecting when an object is near to the sensor, but they are more expensive.

Web links

• Manufacturer’s data sheet for Rapid 82-1020 Hall effect sensor
• Hall effect interactive tutorial

Return to list of datasheets

Infrared photo module - Input Unit

What does it do?

This unit is used for remote control using infrared over a distance of up to about 10m. Depending on the circuit details, it can provide simple on-off operation, or it can receive data from a hand-held remote control or a PIC.

How does it operate?

Circuit for detecting any IR pulses Click on the circuit diagram to download a Livewire file of the circuit that you can investigate and add to your own circuit. The infrared photo module in the Livewire circuit is a socket with the same pin arrangements as the actual detector, not a functioning detector.

Hand-held infrared remote controls used for TV, hi-fi’s etc. send out bursts of pulses of infrared at 38kHz. Infrared photo modules are specially ‘tuned’ to pick up these pulses. These remote controls produce bursts of pulses of infrared at 38kHz. When the detector receives these, its output signal falls to 0V i.e. a digital low (when there are no infrared pulses the signal from the detector is a digital high). The other components in the circuit smooth out the bursts, so the output signal from the subsystem stays low while any button on the remote control is pressed. The output signal can be fed to a further subsystem, such as a PIC or a CMOS 4000B IC.

Circuit for use with Sony IR remote control Click on the circuit diagram to download a Livewire file of the circuit that you can investigate and add to your own circuit. The infrared photo module in the Livewire circuit is a socket with the same pin arrangements as the actual detector, not a functioning detector.

The circuit on the left uses the same infrared photo module, but it is able to receive data (not just a single on/off signal) and needs to be linked to a PICAXE microcontroller. The signal can come from either a Sony TV remote control, or it can be generated by a PICAXE 08M microcontroller. A special command called ‘InfraIn’ in PIC Logicator and PICAXE Program Editor is used to check the data. The InfraIn command waits until it receives a code from the infrared detector. This happens when a button is pressed on the TV remote control (or a code is sent from a PICAXE 08M). So pressing different buttons on the TV remote can give different results on the receiving PIC. Full details of the operation of this system are given on this web site.

Possible applications

• Low cost remote control at a distance of up to about 10m
• Detecting a moving object by checking if it has interrupted an IR beam

Making

Testing

To test the circuit for detecting any IR pulses, make sure that the signal going out (on the green PCB track) is high when no IR signal is being transmitted, and goes low when a button on the remote IR control is pressed.

To test the circuit for use with a Sony TV IR remote control, make sure that the signal going out (on the green PCB track) is high when no IR signal is being transmitted. Connect a PICAXE microcontroller to the detector. Write a short test program to turn an output device off and on when the correct IR code is detected. Extend this to test the response to different buttons.

Fault finding

If there is a fault, check that:

• The detector is the right way round
• The voltage on pin 2 of the IR photo module is low (0V)
• The voltage on pin 3 is high (the supply voltage)
• The capacitor and photo module are the right way round
• The diode (if used) is the right way round

If there is a fault, check the tracks and solder joints.

Alternatives

• Remote control can be achieved with the Rapid 4-bit IR remote control and the TEP Infrared remote control system. These are more expensive than the systems described here, but they can be used with any PIC or other integrated circuits.
• A high power IR LED and detector can provide remote control. This is inexpensive, but will only work reliably over about half a meter and is disturbed by bright light.
• Remote control can also be achieved with radio which has a longer range and can pass through walls and doors. But this is more complicated and expensive.
• Remote control can also be achieved with ultrasound

Web links

• Applications of the infrared photomodule are discussed in the section of this web site on electronic communication
• Manufacturer’s data sheet for HRM138BB5100
• Manufacturer’s data sheet for HRM538BB5100

Keypad - Input Unit

What does it do?

A keypad is used for data entry. It needs to be used with a PIC.

How does it operate?

The keys on a keypad form a matrix of push-to-make switches. This data sheet gives details of the Rapid 78-0305 keypad.

There are 12 push switches, arranged on a matrix of four rows and three columns. These are connected to seven pads at the bottom of the keypad.

If no key is pressed there is no electrical contact between the rows and the columns.

When a key is pressed it makes an electrical connection between the row and the column that it is on.

So, for example, if the key labelled ‘6’ is pressed, this makes electrical contact between row 2 (connected to pad 7) and column 3 (connected to pad 5).

So, if key ‘6’ is pressed this creates an electrical connection between pad 7 and pad 5.

The connections are:

Pad	is connected to
1	column 2
2	row 1
3	column 1
4	row 4
5	column 3
6	row 3
7	row 2

Keypad and PIC connections

Keypads are designed for use with PICs. The four rows of the keypad are connected to four of the output signals from the PIC. The three columns of the keypad are connected to three of the input signals to the PIC. The circuit diagram on the left shows the kinds of connections that might be used but the actual pins used on the PIC depend on the PIC and software used.

The PIC detects which key is pressed by ‘scanning’ the keypad. The principle involved is shown in the section of subroutine. First the PIC sets the signal going to Row 1 high (a digital ‘1’). All the other Rows are made low (a digital ‘0’). Then it tests the input signals coming from each Column in turn. If any of the Column signals is high, this means that the corresponding key in Row 1 has been pressed. If a key has been pressed the software sets a variable (‘A’ in the example) to the value of the key pressed and then returns from the subroutine. After testing the three Columns, the PIC then sets the signal going to Row 2 high and again tests the three columns. In the case of the ‘*’ and the ‘#’ keys, the variable can be given other values (such as 10 and 11). The variable should also be set to another value (such as 13) if none of the keys are being pressed. The main program can then check the value of the variable to find the key that has been pressed. Because a PIC works so quickly the entire scanning process can be completed in a very short time (about a hundredth of a second).

Possible applications

• Entering data into a PIC-based system

Making

The keypad can be mounted on the case, with wires connecting it to the PCB, or it can be mounted directly on the PCB (which gives a more reliable connection).

If it is PCB-mounted, double sided terminal pins can be used to make connections between the PCB and the pads on the keypad (the holes in the keypad pads need to be slightly enlarged to allow the terminal pins to fit).

Testing

Use a multimeter on the resistance setting to check that the columns and rows are connected in the way that you expect when the keys are pressed.

After connecting the keypad, the PIC and an output device, write a test program to test just one switch. Then write a set of programs each of which test a different row.

Fault finding

If there is a fault, check:

• the row and column connections are as expected;
• the connections to the PIC signals and the software correspond;
• the tracks and solder joints.

Alternatives

• If only a few switches are needed they can be each tested separately (this makes the software simpler but uses more PIC signals)
• Another way of entering data into a PIC is to use a Rotation Sensor / Voltage Unit and to read its value using an Analogue to Digital Converters (A to D). This is simpler, but cannot be used to input precise data.

Web links

• Manufacturer’s data sheet for Rapid 78-0305 keypad

Light Sensor - Input Sensor

What does it do?

The light sensor is used to detect changes in the amount of light in its surroundings.

How does it operate?

The usual light sensor circuit gives a low voltage when it is dark and a high voltage when the light is bright.

The light sensor can be followed by a subsystem that processes analogue signals. If the changes in the light level are small and the signal is to be fed to a digital subsystem then the light sensor needs to be followed by a comparator or Schmitt inverter.

Possible applications

• System to check if it is day or night
• An object detector (object blocks light beam)

Making

How part of the PCB might look

The PCB shows the light sensor.

In the dark sensor the position of the two components is reversed.

Testing

Make sure that the signal going out (on the green PCB track) changes from high to low when the light level changes from bright to dark.

In the case of the dark sensor, the signal should be high when it is dark.

How part of the PCB might look

Fault finding

If there is a fault, check that:

• The variable resistor is at its mid point.
• The resistance of the LDR changes when the light level changes (if not, replace it).
• The resistance of the variable resistor changes when it is adjusted (if not, replace it).

If there is a fault, check the tracks and solder joints.

Alternatives

• There are several alternative types of LDR. The traditional type is the ORP12, but smaller ones are cheaper (but may need a higher variable resistor)
• Photodetectors (phototransistors, photodarlingtons and photodiodes) are smaller and cost less.
• Modulated infrared (IR) receivers are useful for detecting the IR signal from TV-style remote controls.
• Optoswitches are useful for detecting when an object is near the sensor, but they are more expensive.
• A Hall effect sensor can be used to detect if a magnet is close

Web links

• Inputs and outputs
• Manufacturer’s data sheet for ORP12

Linear temperature sensor - Input Unit

What does it do?

Linear temperature sensors are used to monitor temperature accurately. They are particularly useful with PICs which have an analogue to digital converter.

How does it operate?

There are a number of linear temperature sensors available. This data sheet gives details of the LM35DZ (the simplest). The output signal voltage from the LM35DZ is proportional to the temperature in degrees C.

The LM35DZ is accurate to at least 2^oC.

If it is being used with a PIC which has an analogue to digital converter (ADC) the PIC needs to be powered from a stabilised 5V supply or a 5V voltage regulator; the voltage signal is measured relative to the power supply voltage – so this needs to be stable.

With a stable 5V supply, the reading from the ADC increases from 0 to 255 (which is 1111 1111 in binary) as the input voltage to the ADC increases from 0V to 5V. So, an increase by one in the reading from the ADC means that the input voltage to the ADC has increased by:

So, for example, if the simple linear temperature sensor circuit is being used and the reading from the ADC is 18, the input voltage to the ADC must be 18 x 20mV = 360mV. Since the output signal voltage of the LM35DZ increases by 10mV for each ^oC, the temperature must be 360/10 = 36^oC.

If the linear temperature sensor circuit for temperatures below 0oC is being used and the reading from the ADC is again 18, the input voltage to the ADC must again be 18 x 20mV = 360mV. However, since there is a voltage of +700mV across the diode, the voltage across the linear temperature sensor must be 360 – 700 = -340mV. Since the output signal voltage of the LM35DZ increases by 10mV for each oC, the temperature must be -340/10 = -34^oC.

Possible applications

• A digital thermometer (with a PIC linked to a 7-segment display)
• An analogue thermometer (with the linear temperature sensor linked to a bargraph display)

Making

Pins of the LM35DZ

How part of the PCB for the simple linear temperature sensor circuit might look

How part of the PCB for the simple linear temperature sensor circuit might look

Make sure that the linear temperature sensor is connected the right way round. If (looking down on the components) the flat side of the package is on the left (as in the PCB diagram) the positive supply +Vs should be connected to the top pin (pin 3) and 0V should be connected to the bottom pin (pin 1).

Testing

Measure the voltage signal going out (on the green PCB track) with a multimeter on the voltage setting at room temperature. Measure room temperature with a thermometer. The voltage signal should be equal to room temperature (in ^oC) x 10mV.

Warm the temperature sensor with your fingers. The output voltage signal should increase by a few tens of mV.

Fault finding

If there is a fault, check that:

• The LM35DZ has been connected the right way round
• The voltage on pin 3 is high (the supply voltage)
• The voltage on pin 1 is low (0V)

If there is a fault, check the tracks and solder joints.

Alternatives

• Temperature Sensor. This uses a thermistor. It is cheaper but less accurate.
• Thermal switch – easy to use, acts like a temperature-activated switch at a fixed temperature.
• Thermocouple – lower cost but needs a fixed reference temperature

Web links

• Manufacturer’s data sheet for LM35DZ

Moisture Sensor - Input Sensor

What does it do?

The moisture sensor is used to detect changes in the wetness of its surroundings.

How does it operate?

The usual moisture sensor circuit gives a low voltage when it is dry and a high voltage when the sensor is wet. The dryness sensor gives a low voltage when it is wet and a high voltage when the sensor is dry.

If the changes in the moisture level are small then the moisture sensor needs to be followed by a comparator.

Possible applications

• Check if plants need watering
• Flood warning system

Making

How part of the PCB might look

Connect the moisture probe to the PCB with wires and a PCB-mounted terminal block.

Testing

Turn the variable resistor to its mid point. Make sure that the signal going out (on the green PCB track) changes from high to low.

Fault finding

If there is a fault, check that:

• The resistance of the moisture probe is high when it is dry and falls to less than 100k when it is wet.
• The resistance of the variable resistor changes when it is turned.

If there is a fault, check the tracks and solder joints.

Alternatives

• Manufactured humidity sensors are available e.g. from Middlesex University Teaching Resources – they are more accurate but more expensive.

Web links

• Inputs and outputs

Movement Sensor - Input Unit

What does it do?

Signals

The movement sensor is a switch. It is ‘on’ (closed) if the sensor is not moved. With the circuit arrangement suggested this gives a low output signal. If the sensor is moved (accelerated) the switch opens and the signal goes high.

How does it operate?

These notes apply to the Rapid movement sensors (Order codes 78-2088 – horizontal sensor, and 78-2104 – vertical).

Both sensors act as a normally closed switch. If they are moved the switch opens. They are quite sensitive to movement – shaking by hand opens the switch.

To work reliably, the sensor needs to be mounted in either the horizontal or vertical position, depending on the type.

Movement sensor circuit

Click on the circuit diagram to download a Livewire file of the circuit that you can investigate and add to your own circuit. Note that there is no Livewire Symbol for a movement sensor - a push to break switch has been used.

In the suggested circuit diagram the movement sensor switch is in the lower part of a potential divider. In the normal switch subsystem the switch is in the upper part of the potential divider.
The result is that, when the sensor is not moved (and the switch is closed) the output signal is low. When the sensor is moved the output goes high.

The signal is only high while the sensor is accelerating, and this is usually only for a short time. Therefore the subsystem to which the sensor is connected must be able to respond quickly to this brief high signal.

Possible applications

• An alarm system (detecting vibrations from an intruder)
• A collision detector on a vehicle
• A hand-held game that responds to movement

Making

How part of the PCB might look

If there is no symbol for the movement sensor in the PCB design package you are using then place pads in suitable positions (0.5” spacing for the horizontal movement sensor). In the example on the left a 0.5” long resistor was used to mimic the dimensions of the movement sensor.

Testing

Make sure that the signal going out (on the green PCB track) briefly changes from low to high when the sensor is moved.

Fault finding

If there is a fault, check that:

• The voltage at the ‘top’ of R1 is high (the supply voltage)
• The voltage at the ‘bottom’ of the sensor is 0V.
• The output signal is normally low

If there is a fault, check the tracks and solder joints.

Alternatives

The tilt switch, microswitch and optoswitch can also be used to detect movement.

Return to list of datasheets

Optoswitch - Input Unit

What does it do?

There are two types of optoswitch – the slotted optoswitch and the reflective optoswitch. Both are used to detect nearby objects. The slotted optoswitch detects when an object is in the slot. The reflective optoswitch detects when a reflective object is near the optoswitch.

How does it operate?

Possible applications

• Detecting the number of rotations (or the speed of rotation) of a wheel with a notch in it
• Detecting when something with white card or aluminium foil attached to it comes near to the sensor

Making

A variety of optosensors are available. Details are given below for four low cost sensors:

• Kingbright KTIR0611S slotted optoswitch
• Kingbright KTIR0221S slotted optoswitch
• Omron EE-SY171 reflective optoswitch
• Kingbright KTIR0821DS slotted optoswitch

Pin arrangements for two slotted optoswitches. The signal from the slotted optoswitch is high when an object blocks the beam.
Kingbright KTIR0221DS. Note that the Anode and Cathode of the LED are marked ‘+’ and ‘E’. The Collector and Emitter of the Darlington pair are marked ‘+’ and ‘D’.	Kingbright KTIR0611S. Note the pip on the bottom surface near to the cathode. There is also a diode symbol and a C/E marked on the top surfaces
Pin arrangements for two reflective optoswitches. The signal from the reflective optoswitch is low when a reflective surface is close to the switch.
Kodenshi SG-2BC. Note the flat side nearer to the collector and cathode.	Omron EE-SY171. Note the dot on the top surface next to the anode.
How part of the PCB for a slotted optoswitch might look	If the Livewire symbol for an opto-isolator is used the default PCB component will be a 6 pin DIL IC. This is not suitable for any of the optoswitches described. The PCB on the left was produced by changing the package for the ‘optoisolator’ to ‘None’ and adding the necessary pads and tracks for the slotted optoswitch manually.

Testing

Make sure that the signal going out (on the green PCB track) changes from high to low when an object is placed in the slot (for the slotted optoswitch) or a reflective object is nearby (for a reflective optosensor).

Fault finding

If there is a fault, check that:

• The voltages on the cathode and the emitter are low (0V)
• The voltage on the anode is about 1.2 to 1.5V
• The optoswitch has been correctly connected

If there is a fault, check the tracks and solder joints.

Alternatives

• A microswitch can be used to detect if an object is nearby but this involves physical contact between the switch and the object
• A reed switch / proximity switch can be used to detect if an object is nearby but this means that a magnet must be attached to the object
• Separate LEDs and light sensors can be used but this is not as reliable
• A Hall effect sensor can be used to detect if a magnet is close

Web links

• Manufacturer’s data sheet for slotted optoswitch Kingbright KTIR0611S – Rapid Order code 58-0942
• Manufacturer's data sheet for slotted optoswitch Kingbright KT1R0221DS - Rapid Order code 58-0310
• Manufacturer’s data sheet for reflective optoswitch Omron EE-SY171 – Rapid Order code 58-0956
• Manufacturer’s data sheet for reflective optoswitch Kingbright KT1R0821DS - Rapid Order code 58-0936

Photodetector - Input Unit

What does it do?

Photodetectors are semiconductor devices which respond to light. They can replace light dependent resistors and have the advantages of lower pollution and smaller size.

How does it operate?

There are several kinds of photodetector.

Photodiodes are similar to normal diodes but, if they are reverse biased, the current through the diode increases with the light level.

Phototransistors (and photodarlingtons) are like ordinary transistors (and Darlington drivers) but the ‘base current’ is produced by the light falling on the device – there in no actual electrical connection to the base.

Possible applications

• Sensing if it is night or day
• Sensing if an object has blocked a beam of light

Making

The pin connections and PCB shown are applicable to both the SFH309F phototransistor (Rapid Electronics Order code 58-0425) and the SFH300-4 (Rapid Electronics Order code 58-0480).

How part of the PCB might look Note – the shorter leg of the phototransistor, and the side with the flat, is the collector. The collector should be connected to the +Vs supply voltage. This is the opposite way round from a LED (hotlink to data sheet), where the shorter leg is the cathode or negative lead.

Testing

Make sure that the signal going out (on the green PCB track) changes from high to low when the photodetector is covered.

Fault finding

If there is a fault, check:

• the voltage on the collector (the leg identified with the flat) is +Vs;
• the value of the resistor
• that the detector has been connected the right way round.

If there is a fault, check the tracks and solder joints.

Alternatives

• Light dependent resistors (LDR) – cheaper, but cause pollution because it contain cadmium. LDRs are also larger.
• A cheaper L-610MP4BT (Rapid Electronics Order code 72-8968) phototransistor is available. However, this produces very, very small currents and can only be used reliably with process subsystems that only need a very low input signal current.

Web links

• Manufacturer’s data sheet for SFH309F

Pulse Unit - Input Block

What does it do?

Turns something on, and then off, and keeps repeating this.

How does it operate?

Possible applications

• Flashing a LED or bulb
• Sounding a buzzer on and off

Making

The inverters can be from a 4069UB inverter, or NAND or NOR gates could be used.

The PCB shows the basic circuit. Several gates in the IC are not used in this simple design; they can be applied in other subsystems. Any unused input pins should be connected to 0V or Vs, to prevent damage by static electricity.

Use a Dual In Line (DIL) socket for the IC. Before inserting the IC, connect the power supply and use a voltmeter to check that the voltage on pin 7 is low (0V) and the voltage on pin 14 is high (the supply voltage).

Connect the negative lead of the capacitor to 0V. Insert the IC the right way round.

Testing

Make sure that the signal going out (on the green PCB track) changes from high to low. Check the time period and make sure it changes with the variable resistor.

Fault finding

If there is a fault, check that:

• The voltage on pin 7 is low (0V)
• The voltage on pin 14 is high (the supply voltage)
• The capacitor and IC are the right way round

If there is a fault, check the tracks and solder joints.

Alternatives

• 555 astable – more accurate time period and variable mark:space ratio but more expensive.
• PICs – more flexible but more expensive.

Web links

• Inputs and outputs
• Manufacturer’s data sheet for 4069 inverter
• Data sheet on CMOS ICs
  

Return to list of datasheets

Quantum Tunnelling Composite (QTC) - Input Unit

What does it do?

QTC comes in the form of sheet, ‘pills’ and cable. QTC sheet can be used to make very low cost touch switches that can be placed on the case of products. QTC ‘pills’ can be used to produce low cost force/pressure sensors whose resistance varies with the applied force.

How does it operate?

Quantum tunnelling composite is a recently developed ‘smart’ material that exhibits extraordinary electrical properties. In its normal state it is an insulator, but when it is compressed it conducts. The material owes its unusual properties (and its name) to a strange phenomenon of quantum mechanics which mean that electrons are able to ‘tunnel’ through some materials i.e. conduct, if their physical structure is slightly changed (by pressure).

QTC sheet

QTC ‘pills’

	Unlike QTC sheet (which switches quite quickly between a high and low resistance), QTC pills are pressure sensitive variable resistors. They can be used as input sensors in electronic systems and the signal from them can go to analogue processing units (such as PICs with analogue to digital converters or amplifiers) to produce a system that responds to weight or force.
	One way to arrange for contact between the pill and the electronic circuit is to use a pair of self adhesive copper tracks, with the QTC ‘pill’ sandwiched between them (it is not necessary to remove the glue). QTC pills are more expensive than QTC sheet and so QTC sheet is a better choice if all that is needed is a simple touch switch.
To represent a QTC pill in a circuit simulation on a computer, use a variable resistor.

Possible applications

QTC sheet can be used to produce:

• low cost touch switches
• switches that can be placed on the surface of products and decorated with graphics

QTC pills can be used to produce:

• input sensors that respond to force or weight
• devices that can control quite high currents, allowing motor speed control using force

Making

QTC sheet switches can be incorporated into the product design for the case, creating adaptable membrane switches. It is best to protect them with a thin foam cover to which graphics can be applied.

When using QTC sheet to make touch switches, it is important to make sure that:

• the QTC sheet is placed with the grey side (the QTC material) next to the copper track and the white side (the insulating plastic layer) away from the copper track
• the stick-on copper track is in good electrical contact with the electronic circuit. Just sticking it onto a terminal or copper track on the circuit does not give reliable electrical contact. It is important to trap the stick-on copper track under a screw head or a hexagonal metal spacer.

Testing

QTC sheet touch switches should be tested by checking the signal going out with a multimeter on the voltage setting. The signal should be low if the switch is not pressed and high when it is pressed.

QTC pills should be tested (before connecting the power supply) using a multimeter on the resistance setting. When no force is applied the resistance should be megohms. Applying a force of about two kg weight should reduce the resistance to less than 100W.

Fault finding

If there is a fault with QTC sheet, check that:

• The sheet is the correct way up
• Nothing (such as the sellotape or graphics) is pressing on the QTC in the gap between the copper tracks
• The copper tracks are not touching each other

If there is a fault with a QTC pill, check that:

• There is good electrical contact between the electronic circuit and the QTC pill
• There is no short circuit across the QTC pill

Alternatives

• Push switches can be used in place of QTC sheet touch switches. They are more expensive but may be easier and clearer to use in some situations.
• Load cells can be used in place of QTC pills, but they are much more expensive.

Web links

• TEP information sheet for QTC
• Peratech are the developers of QTC and have data sheets on:
• QTC pills
• QTC sheet
• Self adhesive copper track is available from Rapid Electronics order code 34-0635
• QTC is available from:
• MUTR
• Economatics

Reed Switch/Proximity Switch - Input Sensor

What does it do?

The reed switch is used to detect the presence of a magnet.

How does it operate?

The contacts of the reed switch are normally open, but close when a magnet is brought near.

In the usual circuit its output signal goes high when a magnet is brought near. The inverted circuit gives a low voltage when a magnet is near.

Possible applications

• Checking if something (with a magnet attached) is near
• Counting on rotating systems
• Checking when a door or window is opened

Making

How part of the PCB might look

Testing

Make sure that the signal going out (on the green PCB track) changes from low to high when a magnet is brought near to the reed switch (or the opposite way round for the inverted circuit).

Fault finding

If there is a fault, check that:

• The resistance of the reed switch component changes from a high resistance to a low resistance when a magnet is brought near.

If there is a fault, check the tracks and solder joints.

Alternatives

• A Hall effect sensor can be used to detect if a magnet is close.
• A microswitch or a light sensor can be used to detect if an object is near
• Optoswitches are useful for detecting when an object is near to the sensor, but they are more expensive.

Web links

• Inputs and outputs

Return to list of datasheets

Rotation Sensor/Voltage Unit - Input Sensor

What does it do?

The rotation sensor uses a potentiometer to sense rotation. It can also be used to produce a variable voltage.

How does it operate?

Click on the circuit diagram to download a Livewire file of the Rotation Sensor that you can investigate and add to your own circuit.	As the potentiometer dial is turned, the resistance of the output signal voltage gradually increases from 0V to the supply voltage Vs. The rotation sensor acts as an input voltage unit. The rotation sensing circuit uses a potentiometer or potential divider: Potentiometers can be mounted on the PCB or on the case.
A potential divider	The resistance across the potentiometer remains constant. However, as the dial is turned the resistance of R1 falls and the resistance of R2 increases. This causes the output voltage at the wiper to rise. Turning the dial the other way reverses this.

Possible applications

• Checking how far something has tilted
• Letting the user vary something by providing a variable voltage to an analogue electronic processing unit

Making

How part of the PCB might look

Testing

Make sure that the voltage signal going out (on the green PCB track) changes as the knob on the potentiometer is turned.

Fault finding

If there is a fault, check that:

• The resistance of the potentiometer between the 0V line and the Output signal line changes from zero to 100k as the knob is turned.

If there is a fault, check the tracks and solder joints.

Alternatives

• Tilt switch – simple but only gives a digital output signal.

Web links

• Inputs and outputs

Return to list of datasheets

Sound Sensor - Input unit

What does it do?

The sound sensor provides an output signal voltage that responds to sound detected by a microphone. It will only respond to loud sounds.

How does it operate?

	If no sound is picked up, the output voltage is low, close to 0V. If the microphone picks up sound the voltage pulses rapidly above 0V in response to the sound wave. The graph shows the kind of output signal that would be produced by a sudden loud noise, such as a hand clap.
Click on the circuit diagram to download a Livewire file of the circuit that you can investigate and add to your own circuit.
Microphone circuit diagram symbol	The symbol used for the microphone in the circuit diagram is a drawing imported into Livewire. Therefore the operation of the microphone cannot be simulated in Livewire.
When a sound wave strikes the microphone it produces a very small a.c. voltage, proportional to the pressure changes produced by the sound wave. This a.c. voltage is amplified by the two operational amplifier circuits. A wide range of microphones is available. The one that has been tested with this circuit diagram was not actually designed as a microphone; it is an uncased piezo transducer (Rapid order code 35-0200). This is low in cost and works effectively. The circuit will respond to loud sounds (such as a hand clap). Its sensitivity can be increased (so that it responds to quite loud music) by reducing the value of the resistor R6 in the circuit diagram from 10k to 1k. However, if the resistor R6 is reduced even further to give higher sensitivity, electrical noise can swamp the output signal and give random output voltages. Because the output signal (like the sound wave) changes very rapidly it is usually necessary to follow the sound sensor with a subsystem that will react quickly to the oscillations, such as a delay unit, a retriggerable monostable, a 555 monostable, a positive latch or a thyristor.

Possible applications

• A toy that responds to hand claps

Making

How part of the PCB might look

The ‘microphone’ (the piezo transducer) is connected to the two pads highlighted in blue.

Testing

This subsystem is difficult to test because of the unusual a.c. output signal. If you have an oscilloscope you can use it to ‘see’ the rapidly varying output signal.

If you do not have access to an oscilloscope then the unit can be tested by adding the next subsystem and checking that its output signal is triggered when there is a loud sound.

Fault finding

If there is a fault, check that:

• The voltage on pin 11 is low (0V)
• The voltage on pin 4 is high (the supply voltage)
• The capacitors and IC are the right way round

If there is a fault, check the tracks and solder joints.

If you have access to an oscilloscope, check the output signal from the first operational amplifier stage (pin 1). When there is a loud noise this should be an a.c. signal of a few hundred mV.

Web links

• Manufacturer’s data sheet for LM 324 operational amplifier
• How microphones work

Switch Unit - Input Unit

What does it do?

The switch subsystem provides a switch that closes when pressed. The output signal from the switch subsystem goes high when the switch is pressed.

How does it operate?

Possible applications

• Providing a signal to make something start or stop
• Sensing an object (microswitch)

Making

How part of the PCB might look

Use a multimeter (on the resistance setting) to check that the switch is working before soldering it in place.

Testing

Make sure that the signal going out (on the green PCB track) changes from low to high when the push switch is pressed.

Fault finding

If there is a fault, check that:

• The voltage at the ‘bottom’ connection of the resistor is low (0V)
• The voltage on the ‘top’ pin of the push switch is high (the supply voltage)

If there is a fault, check the tracks and solder joints.

Alternatives

• A slide switch or a microswitch can be used in place of the push switch
• Quantum Tunnelling Composite (QTC) is a recently developed material that can be used to make up low cost switches.
• Optoswitches are useful for detecting when an object is near to the sensor, but they are more expensive.
• Some push switches are designed for mounting on the PCB; some are ‘panel-mounted’ (for mounting on the case).
• Switches come in a wide range of shapes and colours.
• Keypads are a matrix of push switches and can be used to enter data. They are more sophisticated and more expensive.
• A Hall effect sensor can be used to detect if a magnet is close.

Web links

• Inputs and outputs

Return to list of datasheets

Temperature Sensor (Thermistor) - Input Sensor

What does it do?

The temperature sensor is used to detect changes in the temperature of its surroundings.

How does it operate?

Usually the temperature sensor produces a voltage signal that increases as the temperature increases. The inverted temperature sensor (cold sensor) produces a voltage signal that increases as the temperature decreases.

If the temperature sensor is being used with a digital process unit then it needs to be followed by a comparator or Schmitt inverter to give a sharp change of signal from low to high.

In the normal circuit, the thermistor is placed in the upper half of the potential divider.  In the inverted circuit, the thermistor is placed in the lower half of the potential divider.

Accurate temperature measurement

By using a precision thermistor e.g. the Rapid 10k thermistor 61-0515, it is possible to measure temperatures to an accuracy of „b2oC. The resistance if the thermistor varies with temperature as shown on the graph below:

If the thermistor is placed in a potential divider with a fixed value 10k resistor (not the variable resistor shown in the 'Normal circuit' above) then, with a 5V supply voltage, the Output signal voltage varies with temperature as shown below. For accurate temperature measurements, the 10k resistor needs to have a +/- 1% tolerance, rather than the usual +/- 5%.

If this output signal voltage is fed to a PIC with an analogue to digital converter (ADC) reading between 0 and 255, the reading on the ADC will look like:

Precise values of resistance, output signal voltage and ADC reading can be found by downloading an Excel spreadsheet from this web site. This makes use of what is called the 'B parameter equation' (explained in an article in Wikipedia).

Using these graphs or the spreadsheet, a monitoring or control system can be designed to respond at an accurately predicted temperature. By using a PIC with an ADC, the software can test if the ADC reading is close to the calculated ADC value for the temperatures of interest and activate an output device. By using a liquid crystal display this approach can be used to produce a digital thermometer.

Note - for precise measurements the thermistor should not be placed in water because water conducts electricity and the apparent resistance of the thermistor will be reduced.

Possible applications

• Fire alarm
• Frost warning
• Digital thermometer

Making

Before soldering it into the PCB, make sure (by testing with a multimeter on the resistance setting) that the resistance of the thermistor falls when it is warmed up.

In the case of the precision thermistor, note that a fixed value resistor needs to be used.

How part of the PCB might look

Testing

Turn the variable resistor to its mid-point.

Make sure that the voltage signal going out from the normal circuit (on the green PCB track) increases when the thermistor is warmed up.

In the case of the inverted circuit the signal voltage should fall as the temperature increases.

Fault finding

If there is a fault, check that:

• The voltage at the 'top' of the thermistor is high (the supply voltage)
• The resistance of the variable resistor changes when it is warmed

If there is a fault, check the tracks and solder joints.

Alternatives

• Thermal switch ¡V easy to use, acts like a temperature-activated switch at a fixed temperature.
• Linear temperature sensor ¡V more accurate but more expensive
• Thermocouple ¡V lower cost but needs a fixed reference temperature

Web links

• Inputs and outputs
• Manufacturer's data sheet for Rapid low-cost thermistors
• Manufacturer's data sheet for Rapid precision thermistors
• Wikipedia atical on thermistors

Return to list of datasheets

Tilt Switch - Input Sensor

What does it do?

The tilt switch is used to detect movement.

How does it operate?

Possible applications

• Checking when a moving part e.g. a car park barrier, has turned through a required angle

Making

How part of the PCB might look

Testing

Make sure that the signal going out (on the green PCB track) changes from low to high when the tilt switch sensor is rotated.

Fault finding

If there is a fault, check that:

• The resistance of the tilt switch sensor changes from a high resistance to a low resistance when it is rotated.

If there is a fault, check the tracks and solder joints.

Alternatives

• Rotation Sensor – more suitable for use by a user but requires comparator for use with a digital system
• Microswitch – can check when an object has rotated to a required position but the switch stops any further rotation.

Web links

• Inputs and outputs

Return to list of datasheets

555 Monostable - Process Unit

What does it do?

The 555 monostable subsystem provides an output signal that is triggered high for a period of time before returning to low.

How does it operate?

Once triggered the output remains high for a short time period afterwards. This time period can be calculated:

where R is in M ohms and C is in µF. For example, if R = 100k (= 0.1M) and C = 22µF then the delay time is equal to 1.1 ´ 0.1 ´ 22 = 2.4s.

So, by choosing different component values and adjusting the variable resistor, the delay time can be varied.

The 555 timer IC works with a d.c. power supply with a voltage between 4.5V and 16V. The 555 timer is able to provide an output current of 100mA and can therefore drive low and medium current output devices directly.

There is one quite subtle point to watch with a 555. Nearly all the digital process ICs discussed on this web site are 4000 series CMOS devices. The 555 timer is not in this family and there can be problems in mixing families. In particular, a 4000 CMOS device with a supply voltage of 5V needs an input signal voltage of at least 3.5V to guarantee that it is recognised as a logic '1' or 'high'.

However, at the same supply voltage, a 555 produces an output signal voltage of typically 3.3V, and it can be as low as 2.75V. In other words, the 'high' from a 555 may not be recognised as a 'high' if it is fed to a 4000 series process subsystem.

The safest way to deal with this is to use a CMOS version of the 555 e.g. the ICM7555 (which is more expensive), or a delay unit or retriggerable delay unit – both of which use 4000 series CMOS.

Capacitor types
Ceramic disc capacitors should not be used for the timing capacitor C. They are not sufficiently stable in capacitance to operate properly for timing. Suitable capacitor types are: silver mica, mylar, polycarbonate, polystyrene, tantalum, or similar types.

Possible applications

• Sounding a warning for a short time
• Keeping a car interior light on for a few seconds after the doors have closed.

Making

Pins of 555 timer IC

How part of the PCB might look

Build and test the unit that will trigger the 555 astable before building the 555 astable.

Use a Dual In Line (DIL) socket for the IC. Before inserting the IC, connect the power supply and use a voltmeter to check that:

• the voltage on pin 1 is low (0V);
• the voltages on pins 4 and 8 are high (the supply voltage);
• the voltage on pin 2 (the blue PCB track) goes high and low in response to the unit that provides the input signal.

Insert the IC the right way round.

Testing

Make sure that the signal going out (on the green PCB track) is low at first (while the input signal is high) and then, when the input signal goes low for a short time, the signal going out goes high for the expected time and then goes low.

Fault finding

If there is a fault, check that:

• The voltage on pin 1 is low (0V)
• The voltage on pins 4 and 8 is high (the supply voltage)

If there is a fault, check the tracks and solder joints.

Alternatives

• Delay Unit – useful if ‘spare’ gates are available but delay time is not so precise and more restricted.
• The retriggerable monostable provides a pair of simple delay units but are more expensive.
• PICs – more flexible but more expensive.

Web Links

• Manufacturer's data sheet for 555 timer IC

Return to list of datasheets

AND gate

What does it do?

Makes something happen when both inputs are activated.

How does it operate?

Possible applications

• Turning an output device on when two sensors are both activated
• Making something flash or beep when a sensor is high

Making

Pins of 4081B

How part of the PCB might look

The PCB shows the basic circuit. Several gates in the IC are not used in this simple design; they can be applied in other subsystems. Any unused input pins should be connected to 0V or Vs, to prevent damage by static electricity.

In the example PCB, the two input signals go to pins 1 and 2, and the output signal comes from pin 3. Any of the other three AND gates in the IC could be used.

Build and test the units that will provide the input signals before adding the AND gate IC.

Use a Dual In Line (DIL) socket for the IC. Before inserting the IC, connect the power supply and use a voltmeter to check that:

• the voltage on pin 7 is low (0V);
• the voltage on pin 14 is high (the supply voltage);
• the voltage on pins 1 and 2 (the blue PCB tracks) goes high and low in response to the units that provide the input signals.

Insert the IC the right way round.

Testing

Make sure that the signal going out (on the green PCB track) follows the AND gate truth table.

Fault finding

If there is a fault, check that:

• The voltage on pin 7 is low (0V)
• The voltage on pin 14 is high (the supply voltage)

If there is a fault, check the tracks and solder joints.

Alternatives

• PICs - more flexible but more expensive

Web links

• Building Blocks
• Manufacturer's data sheet for 4081B
• Data sheet on CMOS ICs

Return to list of datasheets

CMOS Integrated Circuits - Process Units

What do they do?

CMOS integrated circuits are widely used as Process units.

How do they operate?

Most of the ‘Process’ units covered in these data sheets are Complementary Metal Oxide Semiconductor (CMOS) 4000B series integrated circuits. The only exceptions are PICs, 555 timers and operational amplifiers. This data sheet gives general information about this important family of ICs.

CMOS ICs are designed so that they all share a set of common features, which are listed below. This makes it easy to connect CMOS ICs to each other.

CMOS ICs are made up of a large number of MOSFETs, all on a single piece of silicon.

Common features of all CMOS ICs

Power supply voltage (Vs)	Any d.c. voltage between 3 and 15V.
Digital ‘high’ or ‘1’ voltage	At least 0.7 ´ Vs. So, for a supply voltage of 5V, the input signal voltage must be more than 3.5V to give a digital ‘high’.
Digital ‘low’ or ‘0’ voltage	Less than 0.3 ´ Vs. So, for a supply voltage of 5V, the input signal voltage must be less than 1.5V to give a digital ‘low’.
Output signal current	Up to 0.12mA is guaranteed if Vs = 5V. In practice, CMOS ICs can usually provide more current (and many published circuit diagrams make use of this) but it is bad practice to design circuits which exceed the manufacturer’s specification.
Input signal current	Up to 1mA.
Operating temperature	Between -40oC and +85oC

Power supply

On nearly all CMOS ICs, if pin 1 (information on IC pins) is at the ‘top left hand corner’, then the negative side of the power supply (0V) is connected to the ‘bottom left-hand corner’ pin. On manufacturers’ data sheets this pin is labelled VSS (standing for the ‘Source’ voltage).

The positive side of the power supply (+Vs) is usually connected to the ‘top right-hand corner’ pin (labelled VDD – ‘Drain’ voltage – on manufacturers’ data sheets).

Using ‘spare’ gates

Most CMOS ICs used for gates contain several gates. For example, the 4011B CMOS IC contains four two-input NAND gates

If a circuit needs, for example, one AND gate, one NAND gate and one inverter it is possible to produce all three of these gates using just one 4011B IC. This saves money and makes the PCB simpler and more reliable.

Pins of 4011B NAND IC

	To make an inverter out of a NAND (or NOR) gate, we simply connect the two input pins together. From the truth table for the NAND gate:
We can see that if both input pins are connected together (so that both input signals are the same) then: if the input signal is ‘0’ then the output signal will be ‘1’; if the input signal is ‘1’ then the output signal will be ‘0’. This is exactly what we would expect from an inverter. In a similar way, from the truth table for a NOR gate, if both its input pins are connected it will also act as an inverter.
If we compare the truth table for a NAND gate: and the truth table for an AND gate: it is clear that the output signal from an AND gate is the inverse of the signal from a NAND gate. So, if we follow a NAND gate with an inverter, the two in combination will act as an AND gate.
	So, if we use a NAND gate, followed by a second NAND gate with its input pins connected (so that it works like an inverter) we have made an AND gate out of two NAND gates. In just the same way, a NOR gate, followed by a second NOR gate with its input pins connected will work as an OR gate.
	So, the circuit on the left will act as: an AND gate – with input signals A and B and output signal X a NAND gate – with input signals C and D and output signal Y, and an inverter – with input signal E and output signal Z
	The PCB on the left is based on the above circuit. As can be seen, by using these techniques, the AND, NAND and inverter can all be produced with just one 4011B IC. The PCB also illustrates that, if this approach is used, it is difficult to avoid using links, unless the tracks are run between IC pins.

Some common CMOS ICs

The pin connections of the CMOS ICs mentioned in these data sheets are shown below.

4001 NOR Gates	4011 NAND Gates
4026 Counter	4029 Counter
4069 Inverters	Pins of 4070B
4071 NOR Gates	4081 AND Gates
40106B Schmitt Inverter

Making

CMOS ICs should not be stored in plastic containers (unless the containers are suitable for electronic components) since this can generate excessive static which can damage them. For the same reason, when designing the PCB, any unused input pins should be connected to 0V or Vs.

A dual in line (DIL) socket should be used with all ICs. This makes testing and repair much easier.

Before inserting the IC, connect the power supply and use a voltmeter to check that:

• the voltage on pin 7 (for most CMOS ICs) is low (0V);
• the voltage on pin 14 (for most CMOS ICs) is high (the supply voltage).

Insert the IC the right way round.

Alternatives

• PICs - more flexible but more expensive

Web links

• Overview of common CMOS ICs
• Common electrical characteristics of the CMOS 4000B family
• Data sheets on common CMOS ICs
• Comprehensive manufacturer’s data sheets for 4000B series CMOS ICs (enter the IC number in the search box)

Return to list of datasheets

Comparator - Process Unit

What does it do?

The comparator provides a large change in signal when the input signal only changes slightly and converts an analogue signal into a digital signal. It ‘compares’ the voltage input signal and the voltage from a potentiometer.

How does it operate?

The comparator circuit uses an operational amplifier, or op amp, to amplify the input signal relative to the reference signal thus forcing the output signal to be either high or low.

Operational amplifiers have two inputs, an inverting input ('-') and a non-inverting input ('+').

A potentiometer or potential divider connected to the inverting input provides control over the reference signal voltage.

There is a wide range of operational amplifiers available. It is important to select one that provides a clear high and low output voltage and can work from a single power supply. The LM324 is inexpensive, contains four operational amplifiers and can work from a d.c. power supply with a voltage anywhere between 3 and 32V. It can give and output current of up to 20mA, and so can drive some low current output devices directly.

Rather than using a potentiometer, the reference voltage can be provided by other analogue input sensors. For example, the light level at two places could be compared using a pair of light sensors, one to provide the input signal and the other to provide the reference signal.

Possible applications

• Converting the signal from an analogue sensor, such as a temperature sensor, to a digital signal to feed to a gate or PIC.

Making

In the example PCB, the input signals goes to pin 3, and the output signal comes from pin 1. Any of the other three comparators in the IC could be used.

Build and test the unit that will provide the input signal before building the comparator.

Use a Dual In Line (DIL) socket for the IC. Before inserting the IC, connect the power supply and use a voltmeter to check that:

• the voltage on pin 11 is low (0V);
• the voltage on pin 4 is high (the supply voltage);
• the voltage on pin 3 (the blue PCB track) varies in response to the unit that provides the input signal.

Insert the IC the right way round.

Testing

Turn the potentiometer to its mid-point. Make sure that the signal going out (on the green PCB track) changes from low to high when the input signal voltage increases above the threshold voltage.

Fault finding

If there is a fault, check that:

• The voltage on pin 11 is low (0V)
• The voltage on pin 4 is high (the supply voltage)
• The threshold voltage (on pin 2) varies between 0V and the supply voltage when the knob on the potentiometer is turned

If there is a fault, check the tracks and solder joints.

Alternatives

• Schmitt inverter can be used instead of a comparator if the input voltage change is reasonably large.

Web links

• Building blocks
• Manufacturer’s data sheet for LM324

Return to list of datasheets

Counter - Process Unit

What does it do?

The counter subsystem counts the number of signal pulses connected to its clock input.

How does it operate?

A range of counter ICs is available. All of them count up (and sometimes down) every time there is a pulse on their ‘clock’ input pin.

The clock pulse needs to be a clean sharp pulse. If it is being produced by any type of mechanical switch it is important to use a debounced switch unit. Otherwise switch bounce can cause multiple counts.

Electrical noise on an analogue signals can trigger counters. This can be removed by using a Schmitt inverter.

Some counters are decade counters, meaning that they count up in the sequence 0, 1, 2, … 8, 9 and then restart at 0. Binary counters use the binary number code and count up in binary: 0000, 0001, 0010, … 1110, 1111 and then restart at 0000.

Some have an input signal that can control whether they count up or down.

Counters are often used with a 7-segment display to show the count value. In most cases this means that a decoder-driver IC needs to be included.

This data sheet provides details of two useful counter ICs; the 4026B and the 4029B.

Possible applications

• Counting objects as they move along a conveyor belt
• Displaying an analogue value, such as a temperature, with a PIC

Making

Pins of 4026B	How part of the 4026 PCB might look
Pins of 4029B	How part of the 4029 PCB might look

Build and test the unit that will provide the input signals before building the counter.

Use a Dual In Line (DIL) socket for the IC. Before inserting the IC, connect the power supply and use a voltmeter to check that:

• the voltage on pin 8 is low (0V);
• the voltage on pin 16 is high (the supply voltage);
• the voltage on input signal pins (the blue PCB tracks) go high and low in response to the units that provides the input signals.

Insert the IC the right way round.

Testing

Make sure that the signals going out (on the green PCB tracks) change from high to low.

Fault finding

If there is a fault, check that:

• The voltage on pin 8 is low (0V)
• The voltage on pin 16 is high (the supply voltage)

If there is a fault, check the tracks and solder joints.

Alternatives

• There are a variety of counter ICs: 4017B – decade counter, 4020 – 14-bit binary counter, 4024 – 7-bit binary counter, 4060 – 12-bit binary counter, 4510 – decade up/down counter, 4516 – binary up/down counter, 4518 – dual decade counter, 4520 – dual binary counter.
• PICs – more flexible but more expensive.

Web links

• Building blocks
• Manufacturer’s data sheet for 4026B
• Manufacturer’s data sheet for 4029B
• Data sheet on CMOS ICs

Return to list of datasheets

Delay - Process Unit

What does it do?

The delay subsystem produces a delay after the input signal goes high.

How does it operate?

Possible applications

• Making a buzzer sound for a fixed length of time after a switch has been pressed.

Making

Pins of 4069B IC

How part of the PCB might look

The PCB shows the basic circuit. Several gates in the IC are not used in this simple design; they can be applied in other subsystems. Any unused input pins should be connected to 0V or Vs, to prevent damage by static electricity.

The transistor used on the PCB is the BC337, but any low power NPN transistor e.g. the BC547B, can be used.

Build and test the unit that will provide the input signal before building the delay unit.

Use a Dual In Line (DIL) socket for the IC. Before inserting the IC, connect the power supply and use a voltmeter to check that:

• the voltage on pin 7 is low (0V);
• the voltage on pin 14 is high (the supply voltage);
• the voltage on pin 1 (the blue PCB track) goes high and low in response to the unit that provides the input signal.

Insert the IC the right way round.

Testing

Turn the variable resistor to its mid-point. Make sure that the signal going out (on the green PCB track – from pin 6) changes from high to low.

Fault finding

If there is a fault, check that:

• The voltage on pin 7 is low (0V)
• The voltage on pin 14 is high (the supply voltage)

If there is a fault, check the tracks and solder joints.

Alternatives

• 555 monostable – more accurate timing but no ‘spare’ gates
• The retriggerable monostable provides a pair of simple, acurate delay units but are more expensive.
• PICs – more flexible and accurate but more expensive.

Web links

• Building blocks
• Manufacturer’s data sheet for 4069 inverter
• Data sheet on CMOS ICs

Return to list of datasheets

Difference Amplifier (or Subtractor) - Process Unit

What does it do?

The difference amplifier is used with two analogue input signals. It gives an analogue output signal which is proportional to the difference between the two input voltages.

How does it operate?

When the two input signals are the same, the output signal will remain steady at half the supply voltage. The output signal will go above or below this steady voltage when one input signal is greater than the other. The output signal is measured relative to a reference signal, which is fixed at half the power supply voltage. (An alternative is to use a pair of power supplies, +Vs and –Vs, and then the reference signal is 0V.)
Click on the circuit diagram to download a Livewire file of the circuit that you can investigate and add to your own circuit.
The difference amplifier circuit uses an operational amplifier, or op amp, to subtract the input signals. Operational amplifiers have two inputs, an inverting input ('-') and a non-inverting input ('+'). The amount of amplification, or gain, can be varied by changing R1, R2, R3 and R4. If (as in the circuit diagram) all four resistors are equal then: R1 must always be the same as R2 and R3 must always be the same as R4. But if all four resistors are not equal then:

The LM324 is a suitable inexpensive IC, contains four operational amplifiers and can work from a d.c. power supply with a voltage anywhere between 3 and 32V.

Possible applications

• Monitoring the difference between two analogue sensors, such as two temperature sensors.

Making

Pins of LM324 IC

How part of the PCB might look

The PCB shows the basic circuit. Three operational amplifiers in the IC are not used in this simple design; they can be applied in other subsystems.

Build and test the unit that will provide the input signals before building the difference amplifier.

Use a Dual In Line (DIL) socket for the IC. Before inserting the IC, connect the power supply and use a voltmeter to check that:

• the voltage on pin 11 is low (0V);
• the voltage on pin 4 is high (the supply voltage);
• the voltages on pins 2 and 3 (connected to the blue PCB tracks) vary in response to the units that provides the input signal.

Insert the IC the right way round.

Testing

Make sure that the signal going out (on the green PCB track) is proportional to the difference between the voltages of the input signals.

Fault finding

If there is a fault, check that:

• The voltage on pin 11 is low (0V)
• The voltage on pin 4 is high (the supply voltage)
• The values of all the resistors is correct

If there is a fault, check the tracks and solder joints.

Alternatives

• The Comparator gives a digital output signal in response to the difference between two analogue voltages
• A PIC can respond to the difference between two analogue voltages provided it has two analogue to digital converter input pins. PICs are more flexible but more expensive.

Web links

• Manufacturer’s data sheet for LM324

Return to list of datasheets

Exclusive OR Gate (XOR/EOR)

What does it do?

Makes something happen when either, but not both, inputs are activated.

How does it operate?

Possible applications

• Turning an output device on when only one out of two sensors is activated

Making

Pins of 4070B

How part of the PCB might look

The PCB shows the basic circuit. Several gates in the IC are not used in this simple design; they can be applied in other subsystems. Any unused input pins should be connected to 0V or Vs, to prevent damage by static electricity.

In the example PCB, the two input signals go to pins 1 and 2, and the output signal comes from pin 3. Any of the other three Exclusive OR gates in the IC could be used.

Build and test the units that will provide the input signals before adding the Exclusive OR gate IC.

Use a Dual In Line (DIL) socket for the IC. Before inserting the IC, connect the power supply and use a voltmeter to check that:

• the voltage on pin 7 is low (0V);
• the voltage on pin 14 is high (the supply voltage);
• the voltages on pins 1 and 2 (the blue PCB tracks) goes high and low in response to the units that provide the input signals.

Insert the IC the right way round.

Testing

Make sure that the signal going out (on the green PCB track) follows the Exclusive OR gate truth table.

Fault finding

If there is a fault, check that:

• The voltage on pin 7 is low (0V)
• The voltage on pin 14 is high (the supply voltage)

If there is a fault, check the tracks and solder joints.

Alternatives

• PICs – more flexible but more expensive.

Web links

• Manufacturer’s data sheet for 4070B
• Data sheet on CMOS ICs

Return to list of datasheets

Inverter (NOT Gate) - Process Unit

What does it do?

The inverter subsystem, also known as a NOT gate, provides an output signal which is opposite to the input signal. When the input signal is high, the output signal is low, and vice versa.

How does it operate?

Possible applications

• Changing the operation of a sensor or a gate, so that something is turned on when, without the inverter, it would have been turned off

Making

Pins of 4069B

How part of the PCB might look

The PCB shows the basic circuit. Several gates in the IC are not used in this simple design; they can be applied in other subsystems. Any unused input pins should be connected to 0V or Vs, to prevent damage by static electricity.

Build and test the unit that will provide the input signal before building the Inverter.

Use a Dual In Line (DIL) socket for the IC. Before inserting the IC, connect the power supply and use a voltmeter to check that:

• the voltage on pin 7 is low (0V);
• the voltage on pin 14 is high (the supply voltage);
• the voltage on pin 1 (the blue PCB track) goes high and low in response to the unit that provides the input signal.

Insert the IC the right way round.

Testing

Make sure that the signal going out (on the green PCB track) changes from high to low when the input signal (on the blue track) changes from low to high.

Fault finding

If there is a fault, check that:

• The voltage on pin 7 is low (0V)
• The voltage on pin 14 is high (the supply voltage)

Check the tracks and solder joints.

Alternatives

• ‘Spare’ NAND and NOR gates on ICs can be turned into Inverters
• PICs – more flexible but more expensive.

Web links

• Building blocks
• Manufacturer’s data sheet for 4069 inverter

Return to list of datasheets

NAND gate - Process Unit

What does it do?

Turns something off when both inputs are activated.

How does it operate?

Possible applications

• Turning an output device off when two sensors are both activated

Making

Pins of 4011B

How part of the PCB might look

The PCB shows the basic circuit. Several gates in the IC are not used in this simple design; they can be applied in other subsystems. Any unused input pins should be connected to 0V or Vs, to prevent damage by static electricity.

In the example PCB, the two input signals go to pins 1 and 2, and the output signal comes from pin 3. Any of the other three NAND gates in the IC could be used.

Build and test the units that will provide the input signals before adding the NAND gate IC.

Use a Dual In Line (DIL) socket for the IC. Before inserting the IC, connect the power supply and use a voltmeter to check that:

• the voltage on pin 7 is low (0V);
• the voltage on pin 14 is high (the supply voltage);
• the voltage on pins 1 and 2 (the blue PCB tracks) goes high and low in response to the units that provide the input signals.

Insert the IC the right way round.

Testing

Make sure that the signal going out (on the green PCB track) follows the NAND gate truth table.

Fault finding

If there is a fault, check that:

• The voltage on pin 7 is low (0V)
• The voltage on pin 14 is high (the supply voltage)

If there is a fault, check the tracks and solder joints.

Alternatives

• PICs – more flexible but more expensive.

Web links

• Building Blocks
• Manufacturer’s data sheet for 4011B
• Data sheet on CMOS ICs

Return to list of datasheets

Negative Latch - Process Unit

What does it do?

The negative latch produces an output signal that goes high and remains high when the input signal has been low. It is useful for turning something on until a second signal switches it off.

How does it operate?

Possible applications

• Keeping an output device turned on until it is switched off by a separate switch.

Making

Pins of 4011 NAND gate IC

How part of the PCB might look

The PCB shows the basic circuit. Several gates in the IC are not used in this simple design; they can be applied in other subsystems. Any unused input pins should be connected to 0V or Vs, to prevent damage by static electricity.

Build and test the unit that will provide the input signal before building the latch.

Use a Dual In Line (DIL) socket for the IC. Before inserting the IC, connect the power supply and use a voltmeter to check that:

• the voltage on pin 7 is low (0V);
• the voltage on pin 14 is high (the supply voltage);
• the voltage on pin 1 (the blue PCB track) goes high and low in response to the unit that provides the input signal.

Add the switch and resistor and test that the signal on pin 6 goes low when the switch is pressed. Insert the IC the right way round.

Testing

Make sure that the signal going out (on the green PCB track) goes high and stays high after a low signal on the input pin.

Fault finding

If there is a fault, check that:

• The voltage on pin 7 is low (0V)
• The voltage on pin 14 is high (the supply voltage)

If there is a fault, check the tracks and solder joints.

Alternatives

• The positive latch – easier to understand
• PICs – more flexible but more expensive.
• The thyristor acts as a combined latch and driver

Web links

• Building blocks
• Manufacturer’s data sheet for 4011B
• Data sheet on CMOS ICs

Return to list of datasheets

Non-Inverting Amplifier - Process Unit

What does it do?

The non-inverting amplifier subsystem is used to amplify an analogue input signal.

How does it operate?

The LM324 is a suitable inexpensive IC, contains four operational amplifiers and can work from a d.c. power supply with a voltage anywhere between 3 and 32V.

Possible applications

• Increasing the change in the voltage from an analogue sensor, such as a thermistor or a light sensor.

Making

Pins of LM324IC

How part of the PCB might look

In the example PCB, the input signals goes to pin 3, and the output signal comes from pin 1. Any of the other three operational amplifiers in the IC could be used.

Build and test the unit that will provide the input signal before building the comparator.

Use a Dual In Line (DIL) socket for the IC. Before inserting the IC, connect the power supply and use a voltmeter to check that:

• the voltage on pin 11 is low (0V);
• the voltage on pin 4 is high (the supply voltage);
• the voltage on pin 3 (the blue PCB track) varies in response to the unit that provides the input signal.

Insert the IC the right way round.

Testing

Turn the potentiometer to its mid-point. Make sure that the signal going out (on the green PCB track) increases as the input signal increases.

Fault finding

If there is a fault, check that:

• The voltage on pin 11 is low (0V)
• The voltage on pin 4 is high (the supply voltage)

If there is a fault, check the tracks and solder joints.

Alternatives

• A PIC with an analogue to digital converter can respond to changes in an analogue voltage; PICs are more flexible but more expensive than a single LM324.

Web links

• Building blocks
• Manufacturer’s data sheet for LM324

Return to list of datasheets

NOR gate - Process Units

What does it do?

Turns something off when either input is activated.

How does it operate?

Possible applications

• Turning an output device off when either of two sensors is activated

Making

Pins of 4001B

How part of the PCB might look

The PCB shows the basic circuit. Several gates in the IC are not used in this simple design; they can be applied in other subsystems. Any unused input pins should be connected to 0V or Vs, to prevent damage by static electricity.

In the example PCB, the two input signals go to pins 1 and 2, and the output signal comes from pin 3. Any of the other three NOR gates in the IC could be used.

Build and test the units that will provide the input signals before adding the NOR gate IC.

Use a Dual In Line (DIL) socket for the IC. Before inserting the IC, connect the power supply and use a voltmeter to check that:

• the voltage on pin 7 is low (0V);
• the voltage on pin 14 is high (the supply voltage);
• the voltage on pins 1 and 2 (the blue PCB tracks) goes high and low in response to the units that provide the input signals.

Insert the IC the right way round.

Testing

Make sure that the signal going out (on the green PCB track) follows the NOR gate truth table.

Fault finding

If there is a fault, check that:

• The voltage on pin 7 is low (0V)
• The voltage on pin 14 is high (the supply voltage)

If there is a fault, check the tracks and solder joints.

Alternatives

• PICs – more flexible but more expensive.

Web links

• Building Blocks
• Manufacturer’s data sheet for 4001B
• Data sheet on CMOS ICs

Return to list of datasheets

OR gate - Process Unit

What does it do?

Turns something on when either input is activated.

How does it operate?

Possible applications

• Turning an output device on when either of two sensors is activated

Making

Pins of 4071B

How part of the PCB might look

The PCB shows the basic circuit. Several gates in the IC are not used in this simple design; they can be applied in other subsystems. Any unused input pins should be connected to 0V or Vs, to prevent damage by static electricity.

In the example PCB, the two input signals go to pins 1 and 2, and the output signal comes from pin 3. Any of the other three OR gates in the IC could be used.

Build and test the units that will provide the input signals before adding the NOR gate IC.

Use a Dual In Line (DIL) socket for the IC. Before inserting the IC, connect the power supply and use a voltmeter to check that:

• the voltage on pin 7 is low (0V);
• the voltage on pin 14 is high (the supply voltage);
• the voltage on pins 1 and 2 (the blue PCB tracks) goes high and low in response to the units that provide the input signals.

Insert the IC the right way round.

Testing

Make sure that the signal going out (on the green PCB track) follows the OR gate truth table.

Fault finding

If there is a fault, check that:

• The voltage on pin 7 is low (0V)
• The voltage on pin 14 is high (the supply voltage)

If there is a fault, check the tracks and solder joints.

Alternatives

• PICs – more flexible but more expensive.

Web links

• Building Blocks
• Manufacturer’s data sheet for 4071B
• Data sheet on CMOS ICs

Return to list of datasheets

PIC Microcontrollers - Process Unit

What does it do?

PIC microcontrollers are general purpose programmable process units.

How do they operate?

PIC microcontrollers are ‘computers on a chip’. They can replace the large number of fixed-function integrated circuits, such as CMOS ICs.

The way in which a PIC microcontroller behaves is controlled by a program. Depending on the program, a PIC can behave like combinations of gates, timers, counters, latches etc., as well as perform other functions that would be very complicated using ‘hard-wired’ logic.

The section on Software Engineering on this web site gives details of PIC programming.

This data sheet gives outline information relevant to PICs used in schools.

Selecting a suitable PIC

There is a potentially bewildering range of PICs available. In practice the selection process is not as complicated as it looks.

• First, decide on the PIC programming system that you will use. The Pedagogy section of this web site discusses some of the options. As is clear from the table below, once the PIC programming system is selected, this reduces the choice of PICs dramatically.
• One factor that may well influence the choice of PIC programming system is whether or not it allows in-circuit programming. In-circuit programming means that the PIC can be programmed from the computer without removing it from the circuit. PICs that are not programmed in-circuit have to be placed in a special programmer (which adds to costs) and then transferred to the circuit (with a good chance of being damaged in the process). PICAXE, EChip and SOLO use pre-prepared PICs. ICON software can program PICs in-circuit that have not been pre-prepared, using a special adaptor.
• Next, decide on the size of PIC needed. PICs come with different numbers of pins: 8, 14, 18, 28 and 40. The more pins a PIC has the more input sensors and output devices that can be used with it. However, the larger the PIC, the more expensive it is and the more complicated the PCB will be. Make sure that the number of input and output pins available is enough for the planned application. Generally speaking, low cost 8-pin PICs are suitable for most work at KS3 and quite a lot of work at KS4. Some GCSE projects will need 18 pin PICs and a few may need 28 pin PICs.
• Older PICs used an external oscillator. More modern PICs have an internal oscillator and therefore have a simpler PCB. An external oscillator is necessary if the timing accuracy needs to be better than 1%.
• If analogue sensors are being used then a PIC with a built-in Analogue to Digital Converter (A to D) is needed.
• Linking a PIC to some input and output devices e.g. Piezo transducers, liquid crystal displays, servo motors, is only possible with a limited range of PICS and PIC programming systems.
• By this stage the choice should be limited to a small number of PICs. The final issue to consider is the amount of memory the PIC will need to store the program. It is not possible to know the amount of memory needed until the program has been written. However, it is usually possible to select a group of PICs which are very similar and that only differ in the amount of memory they have. In this way, all the earlier stages of development (designing the PCB, writing the program) can be done and the final selection of PIC left to a late stage. Generally speaking, the more memory a PIC has the more expensive it is.

Once the PIC is selected it is important to be aware of the supply voltage needed. It is convenient, where possible, to use the same power supply for all the electronic subsystems. Most PICs can provide an output current of 25mA per pin, but some have a lower limit. PICs are therefore able to drive some low to medium power devices, such as piezo transducers, some LEDs and some buzzers, without the need for a driver.

PICs available

Notes

• Circuit information depends on the type of PIC. For details see suppliers’ web sites listed below.
• To reduce electrical noise (which can cause a PIC to ‘crash’) place a small capacitor, such as a 100nF polyester, across the power supply rails, as close as possible to the PIC.
• The maximum current an output pin of a PIC can supply is 25mA. In the case of some PICs there is a limit of 100mA on the total current that can be supplied from all 8 output pins – limiting the average current per pin to 12.5mA in these cases. Some output devices that require modest current (LEDS, piezo transducers, some buzzers) can therefore be driven directly by PICs. If an output device needs more current than the PIC can supply then a driver needs to be added.
• All PICAXE microcontrollers will run with a power supply in the range 3 to 5.5V d.c. after they have been programmed. However, a power supply of 4.5 – 5.5V should be used to ensure that the program downloads reliably from the computer.
• The TEP Phone PIC Programmer uses the 16F84 PIC, but these need to have been pre-programmed by MUTR.

Possible applications

• PICs can be used in place of one or more ‘process’ ICs.

Making

Use a Dual In Line (DIL) socket for the PIC. Before inserting the PIC, connect the power supply and use a voltmeter to check that:

• the voltage on the 0V terminal is low;
• the voltage on the +V terminal is high;
• the voltage on the input pins goes high and low in response to the units that provide the input signals.

Insert the PIC the right way round.

Testing

Solder in place the components needed for just one output device. Write a short test program that pulses this output device on and off. When you are sure this is working properly, add and test each output device in this way one at a time.

When all the output devices are working, solder in place the components for one input sensor. Write a short test program that turns one of the output devices on and off when the signal from this input device is high or low. When you are sure the input sensor is working properly, add and test each input sensor one at a time.

Fault finding

If there is a fault, check that:

• the voltage on the 0V pin is low;
• the voltage on the +V pin is high.

If there is a fault, check the tracks and solder joints.

Alternatives

• Separate ‘hardwired’ ICs can be used. This can be a cheaper solution if the circuit is very simple but it is not possible to modify the way the system works after it has been made.

Web links

Suppliers (many of these websites include information about the relevant PICs plus circuit and PCB information):

• Control Insight
• Crocodile Technology
• EChip
• Flowol
• ICON
• PIC Logicator
• Programming Editor
• TEP Chip Factory
• TEP Phone Chip
• PICs are manufactured by Microchip. Very detailed data sheets are available.

Further circuits and PCB information:

• PCBs for PICAXE designed by John Cook:
• Four downloadable PCB Wizard PICAXE PCBs
• 'Practical PICAXE' information sheets
• Ten real-PCB PICAXE files
• PCB Design and Make
• PCB files

Return to list of datasheets

Positive Latch - Process Unit

What does it do?

The positive latch produces an output signal that goes high and remains high when the input signal has been high. It is useful for turning something on until a second signal switches it off.

How does it operate?

Possible applications

• Keeping an output device turned on until it is switched off by a separate switch.

Making

Pins of 4001 NAND gate IC

How part of the PCB might look

The PCB shows the basic circuit. Several gates in the IC are not used in this simple design; they can be applied in other subsystems. Any unused input pins should be connected to 0V or Vs, to prevent damage by static electricity.

Build and test the unit that will provide the input signal before building the latch.

Use a Dual In Line (DIL) socket for the IC. Before inserting the IC, connect the power supply and use a voltmeter to check that:

• the voltage on pin 7 is low (0V);
• the voltage on pin 14 is high (the supply voltage);
• the voltage on pin 1 (the blue PCB track) goes high and low in response to the unit that provides the input signal.

Add the switch and resistor and test that the signal on pin 6 goes low when the switch is pressed. Insert the IC the right way round.

Testing

Make sure that the signal going out (on the green PCB track) goes high and stays high after a high signal on the input pin.

Fault finding

If there is a fault, check that:

• The voltage on pin 7 is low (0V)
• The voltage on pin 14 is high (the supply voltage)

If there is a fault, check the tracks and solder joints.

Alternatives

• The negative latch – may be useful if there are ‘spare’ NAND gates but it is more difficult to understand
• PICs – more flexible but more expensive.
• The thyristor acts as a combined latch and driver

Web links

• Building blocks
• Manufacturer’s data sheet for 4001B
• Data sheet on CMOS ICs

Return to list of datasheets

Retriggerable Monostable - Process Unit

What does it do?

The retriggerable monostable subsystem produces a delay after the input signal goes high.

How does it operate?

Possible applications

• Making a buzzer sound for a fixed length of time after a switch has been pressed.

Making

Pins of 4098B IC

How part of the PCB might look

Build and test the unit that will provide the input signal before building the retriggerable monostable unit.

Use a Dual In Line (DIL) socket for the IC. Before inserting the IC, connect the power supply and use a voltmeter to check that:

• the voltage on pins 3, 4 and 8 is low (0V);
• the voltage on pins 5, 11, 13 and 16 is high (the supply voltage);
• the voltage on pin 12 (the green PCB track) goes high and low in response to the unit that provides the input signal.

Insert the IC the right way round.

Testing

Make sure that the signal going out (on the blue PCB track – from pin 10) changes from high to low.

Fault finding

If there is a fault, check that:

• The voltage on pins 3, 4 and 8 is low (0V)
• The voltage on pins 13 and 16 is high (the supply voltage)

If there is a fault, check the tracks and solder joints.

Alternatives

• The delay unit – is triggered as long as the input signal is high. Timing is not as accurate. There are no ‘spare’ monostables on the IC.
• 555 monostable – the voltage levels are not reliably compatible with CMOS ICs
• PICs – more flexible and accurate but more expensive.

Web links

• Manufacturer’s data sheet for 4098B monostable
• Data sheet on CMOS ICs

Schmitt Inverter (Schmitt NOT Gate) - Process Unit

What does it do?

The Schmitt inverter subsystem, also known as a Schmitt NOT gate, provides an output signal which is opposite to the input signal. The Schmitt inverter is ideal for converting analogue signals into digital signals.

How does it operate?

Possible applications

• Removing the effects of noise due to fluorescent lighting on a light sensor
• Debouncing mechanical switches.
• Producing a digital signal from an analogue sensor, e.g. a thermistor or light sensor.

Making

Pins of 40106B Schmitt inverter

How part of the PCB might look

The PCB shows the basic circuit. Several gates in the IC are not used in this simple design; they can be applied in other subsystems. Any unused input pins should be connected to 0V or Vs, to prevent damage by static electricity.

Build and test the unit that will provide the input signal before building the Inverter.

Use a Dual In Line (DIL) socket for the IC. Before inserting the IC, connect the power supply and use a voltmeter to check that:

• the voltage on pin 7 is low (0V);
• the voltage on pin 14 is high (the supply voltage);
• the voltage on pin 1 (the blue PCB track) goes high and low in response to the unit that provides the input signal.

Insert the IC the right way round.

Testing

Make sure that the signal going out (on the green PCB track) changes from high to low.

Fault finding

If there is a fault, check that:

• The voltage on pin 7 is low (0V)
• The voltage on pin 14 is high (the supply voltage)

If there is a fault, check the tracks and solder joints.

Alternatives

• An ordinary inverter provides simple inverting action
• A comparator can be used for converting an analogue signal to digital
• PICs – more flexible but more expensive.

Web links

• Building blocks
• Manufacturer’s data sheet for 40106B
• Data sheet on CMOS ICs

Return to list of datasheets

Summing Amplifier - Process Unit

What does it do?

The summing amplifier subsystem is used to add two analogue input signals together.

How does it operate?

The LM324 is a suitable inexpensive IC, contains four operational amplifiers and can work from a d.c. power supply with a voltage anywhere between 3 and 32V.

Possible applications

• Combining the signal from two analogue sensors, such as two temperature sensors.

Making

Pins of LM324 IC

How part of the PCB might look

The PCB shows the basic circuit. Three operational amplifiers in the IC are not used in this simple design; they can be applied in other subsystems.

Build and test the unit that will provide the input signals before building the difference amplifier.

Use a Dual In Line (DIL) socket for the IC. Before inserting the IC, connect the power supply and use a voltmeter to check that:

• the voltage on pin 11 is low (0V);
• the voltage on pin 4 is high (the supply voltage);
• the voltages connected to R1 and R2 (the blue PCB tracks) vary in response to the units that provides the input signal.

Insert the IC the right way round.

Testing

Make sure that the signal going out (on the green PCB track) is proportional to the sum of the voltages of the input signals.

Fault finding

If there is a fault, check that:

• The voltage on pin 11 is low (0V)
• The voltage on pin 4 is high (the supply voltage)
• The values of all the resistors are correct

If there is a fault, check the tracks and solder joints.

Alternatives

• PICs can respond to the sum of two analogue voltages provided they have two analogue to digital converter input pins. They are more flexible but more expensive.

Web links

• Manufacturer’s data sheet for LM324

Return to list of datasheets

Darlington Driver - Driver Unit

What does it do?

The Darlington driver subsystem is an electronic switch that provides an output signal powerful enough to drive output subsystems requiring high current.

How does it operate?

Possible applications

• Providing enough current to drive an output device that needs a relatively high current, such as a motor or solenoid.

Making

Pins of the BCX38B, BCX38C and MPSA13

How part of the PCB might look

When using a Darlington Driver it is important to be clear about which pin is the collector (C), which is the base (B) and which is the emitter (E).

Manufacturer’s data sheets usually show the pins viewed from underneath (unlike IC pins, where the view shown is from above).

The PCB shows the basic circuit.

Build and test the unit that will provide the input signal before building the Darlington Driver.

Testing

Make sure that the signal going out (on the green PCB track) is the inverse of the input signal (on the blue PCB track).

Fault finding

If there is a fault, check that the Darlington Driver is connected the right way round. Then check the resistor values. Check the tracks and solder joints.

Alternatives

• Transistor – cheaper but provides less current
• MOSFET (Transducer driver) – can provide even more current but is more expensive
• The L293D driver can drive high current output devices and reverse two motors but is more expensive.

Web links

• Building blocks
• Manufacturer’s data sheet for BCX38A, BCX38B and BCX38C
• Manufacturer’s data sheet for ULN 2803A

Return to list of datasheets

L293D driver - Driver Unit

What does it do?

The L293D driver subsystem is particularly useful for use with d.c. motors because it can control two motors and can drive them forwards and backwards.

How does it operate?

When current is flowing out of the L293D this is called ‘sourcing’. When current is flowing into the L293D this is called ‘sinking’.

Possible applications

• Driving a pair of motors in forward and reverse
• Driving one motor in forward and reverse, and switching on up to two other output devices, such as a bulb or a buzzer
• Controlling the movement of a stepper motor.

Making

Build and test the units that will provide the input signals before building the L293D driver.

Testing

Make sure that the signals going out (on the green PCB tracks) are the same (high or low) as the input signals (on the blue PCB tracks).

Fault finding

If there is a fault, check that the L293D is connected the right way round. Check the tracks and solder joints.

Alternatives

• Relay – a double pole double throw relay can be used to reverse a motor, but it needs more components
• Transistor – cheaper but cannot reverse a motor and provides less current
• Darlington driver – cannot reverse a motor
• MOSFET (Transducer driver) – cannot reverse a motor

Web links

• Manufacturer’s data sheet for L293D

MOSFET (Transducer Driver) - Driver Unit

What does it do?

The MOSFET driver subsystem is an electronic switch that provides an output signal powerful enough to drive output subsystems requiring very high current.

How does it operate?

Possible applications

• Providing enough current to drive an output device that needs a relatively high current, such as a motor or solenoid.

Making

Pins of the ZVN4306A MOSFET	Pins of the VNP10N06 MOFSET
Pins of the BS170 MOSFET	How part of the PCB might look using the BS170

When using a MOSFET it is important to be clear about which pin is the drain (D), which is the gate (G) (the equivalent pin is labelled ‘Input’ in the VNP10N06) and which is the source (S). The drain in connected to the load, the gate to the input signal and the source to 0V.

Manufacturer’s data sheets usually show the pins viewed from underneath (unlike IC pins, where the view shown is from above).

Build and test the unit that will provide the input signal before building the MOSFET.

Testing

Make sure that the signal going out (on the green PCB track) is the inverse of the input signal (on the blue PCB track).

Fault finding

If there is a fault, check that the MOSFET is connected the right way round. Then check the resistor value. Check the tracks and solder joints.

Alternatives

• Transistor – cheaper but provides less current
• Darlington Driver – usually cheaper than the MOSFET but cannot provide such a large current
• The L293D driver can drive high current output devices and reverse two motors but is more expensive.

Web links

• Building blocks
• Manufacturer’s data sheet for BS170 MOSFET
• Manufacturer’s data sheet for IRF 630
• Manufacturer’s data sheet for VNP10N06

Return to list of datasheets

Relay - Driver Unit

What does it do?

The relay subsystem is an electrically-operated switch. It requires a separate electrical supply to provide power to an output device. It is often used for reversing motors.

How does it operate?

A few relays need relatively low currents and can be driven directly from a PIC, 555 Timer IC or LM324 op-amp. In these cases the relay coil is connected to the input signal and to 0V.

Possible applications

• Reversing a motor
• Providing electrical isolation between a noisy output device (such as a motor) and the processing electronics.
• Controlling a low voltage a.c. output device, e.g. a low voltage halogen bulb (hotlink to bulb data sheet, section that refers to halogen bulb)

Making

Pins of the Rapid 60-0100 DPDT relay

How part of the PCB might look

The diagram shows the pin arrangements and numbering for the Rapid 60-0100 DPDT relay. Note the unusual pin labelling system. The PCB shows the basic circuit. The separate power supply and output device would be connected to the six upper pins.

Build and test the driver unit that will provide the input signal before building the relay.

Use a 16-pin Dual In Line (DIL) socket for the relay. Before inserting the relay, connect the power supply and use a voltmeter to check that:

• the voltage on pin ‘b’ is high (the supply voltage);
• the voltage on pin ‘a’ (the blue PCB track) goes high and low in response to the driver unit that provides the input signal.

Insert the relay the right way round.

Testing

Use a multimeter to test the resistance between the switch contacts and make sure that their resistance changes from high to low when the coil is switched on and off.

Fault finding

If there is a fault, check that:

• The voltage on pin ‘b’ is high
• The relay has been correctly inserted

If there is a fault, check the tracks and solder joints.

Alternatives

• A pair of SPDT relays can be used to provide forward, reverse, stop and start for a motor – this has the advantage of providing a better ‘brake’ for a motor but is more expensive.
• A L293D IC can also be used to provide forward, reverse, stop and start for two motors – this has the advantage of providing a better ‘brake’ for a motor, but is more expensive.
• If the relay is being used to provide electrical isolation from noise, an alternative is the opto-isolator.

Web links

• Building blocks
• Manufacturer's data sheet for the Rapid 60-0100 DPDT relay
• Manufacturer's data sheet for the Rapid 60-0785 SPDT relay

Return to list of datasheets

Transistor - Driver Unit

What does it do?

The transistor driver subsystem is an electronic switch that provides an output signal powerful enough to drive output subsystems requiring medium current.

How does it operate?

Possible applications

• Boosting the current from a process unit to drive an output device that needs a medium current, such as a buzzer, bulb or LED (PICs can provide enough current for LEDs, but CMOS ICs cannot).

Making

Pins of the BC547B

How part of the PCB might look

When using a transistor it is important to be clear about which pin is the collector (C), which is the base (B) and which is the emitter (E). The PCB diagram shows the connections for the BC547B.

Manufacturer’s data sheets usually show the pins viewed from underneath (unlike IC pins, where the view shown is from above).

Build and test the unit that will provide the input signal before building the Transistor Driver.

Testing

Make sure that the signal going out (on the green PCB track) is the inverse of the input signal (on the blue PCB track).

Fault finding

If there is a fault, check that the transistor is connected the right way round. Then check the resistor values. Check the tracks and solder joints.

Alternatives

• Darlington driver – provides more current but is more expensive
• MOSFET (Transducer driver) – can provide even more current but is more expensive
• The L293D driver can drive high current output devices and reverse two motors but is more expensive.

Web links

• Building blocks
• Manufacturer’s data sheet for BC547B

Return to list of datasheets

Thyristor - Driver Unit

What does it do?

A thyristor is used to drive a load. It is switched on by applying a positive voltage to its input pin (the ‘gate’).

How does it operate?

It is important to select a thyristor which can provide a maximum current greater than the current needed by the Output device. Otherwise the thyristor could be damaged. A variety of thyristors are available. Some low cost examples are:

Thyristors are also known as Silicon Controlled Rectifiers (SCRs).

Possible applications

• A ‘smart’ steady hand game or other game of skill that sounds a buzzer if the wire is touched and keeps sounding it until a push-to-break switch is pressed.

Making

The PCB shows the output device connected via a 2-way PCB terminal.

Testing

Before adding the components, make sure that the signal coming in (on the blue PCB track) changes from high to low.

Fault finding

If there is a fault, check that:

• The voltage on the cathode pin is low (0V)
• The input signal voltage on the gate is the same as the signal from the previous subsystem
• The thyristor is the right way round

If there is a fault, check the tracks and solder joints.

Alternatives

• A latch, followed by a driver can be used to give a similar action, but this produces a more complicated circuit.

Web links

• Manufacturer’s data sheet for 2N5060 - Rapid code 47-3266
• Manufacturer’s data sheet for C106D1 – Rapid code 47-3330

7 Segment Display - Output Device

What does it do?

The 7-segment display is used to display numbers.

How does it operate?

Possible applications

• Showing the number of objects that have been counted
• Showing the time in seconds

Making

How part of the PCB might look

Pin connections of the Rapid 57-0127 common cathode seven segment display

The diagram above shows the relationship between the segments and the IC pins for the 7 segment display (Rapid order code 57-0127) shown in the PCB diagram. The Decimal Point (DP) is not usually used.

Note that different 7 segment displays have different pin arrangements.

Build and test the unit that will provide the input signals before building the 7 segment display.

It is not usually possible to use a Dual In Line (DIL) socket for the display because of its shape and size. Use strips of Single In Line (SIL) sockets, snapped to the correct length.

Before inserting the IC, connect the power supply and use a voltmeter to check that:

• the voltage on pin 8 is low (0V);
• the voltage on the input pins (the blue PCB tracks) go high and low in response to the unit that provides the input signals.

Before inserting the 7 segment display, make sure that you have identified pin 1.

Testing

Make sure that the display shows all the digits from 0 to 9 correctly.

Fault finding

If there is a fault, investigate if it is isolated to one or two segments; if so, check all the tracks and solder joints for the faulty segment(s).

Alternatives

• Liquid crystal displays (LCDs) can be used to display digits and also letters, but they are more expensive.
• Bar graph displays can be used to give a visual indication of a how something varies, but they do not give a numerical reading.

Web links

• Building blocks
• Manufacturer's data sheet for Rapid 57-0127 display
• Manufacturer’s data sheet for 4511B decoder driver

Return to list of datasheets

Bar graph display (and driver) - Output Device

What does it do?

A bargraph display is usually used to give a visual indication of an analogue voltage signal.

How does it operate?

In the circuit diagram R1 and R2 form a potential divider, to scale down the Input signal by a factor of about four. So, an Input signal voltage of 5V gives a voltage at pin 5 of the driver IC of 1.25V, producing ‘full scale’ on the LEDs. To give a ‘full scale’ reading for a different Input signal voltage the values of R1 and R2 would need to be changed.

The resistor R3 regulates the current in the LEDs. The value of 1k used gives a LED current of about 12mA.

Possible applications

Showing variations in analogue quantities, such as:

• temperature;
• speed;
• light level;
• sound volume.

Making

Pin diagram for the LM3914 Bargraph Display Driver	Pin diagram for the Rapid 55-0190 Bargraph Display
	Pin 1 is identified by small flat on the corner of the IC package, not by the usual dot or notch
	The PCB for a driver unit and bargraph display might look similar to the example on the left. Small IC pads and narrow tracks have been used to allow the tracks to be run between the pads. This reduces the number of wire links needed but needs careful soldering. The driver IC and the bargraph display are both mounted with pin 1 at the bottom right (rather than the top left, as in the usual arrangement). This is done so that the first LED that lights is at the bottom, and to simplify the tracks.

Build and test the unit that will provide the analogue input signal before adding the bargraph driver and display.

Testing

Check that:

• the voltage on pins 2, 4 and 8 of the driver IC is at 0V
• the voltage on pins 3 and 9 of the driver IC is at +Vs
• the voltage on pins 1 to 10 of the bargraph display is at +Vs
• as the analogue Input signal is varied, the voltage on pin 5 of the driver IC is about a quarter of the analogue Input signal
• as the analogue Input signal is varied, the number of illuminated LEDs on the bargraph display changes

Fault finding

If there is a fault, check all the connections and soldering.

Alternatives

• The LM3915 and the LM3916 are very similar to the LM3914, but they are more suitable for displaying sound volumes (like the familiar VU meters on hi-fi systems) because they show the information on a logarithmic scale
• An analogue voltage meter can be used to display voltage levels but this is probably more expensive.
• Separate LEDs (perhaps in a different arrangement) could be used in place of the bargraph display, but this has the disadvantage of making the wiring more complicated.
• Larger (12 LED) displays and drivers are available.
• A PIC could be used in place of the driver IC, giving more control over the display. An 18 pin PIC usually has 8 output signals, so this could not control all 10 LEDs.

Web links

• Manufacturer’s data sheet for LM3914N

Bulb (or lamp) - Output Device

What does it do?

The bulb subsystem converts the input signal into light.

How does it operate?

Possible applications

• Providing illumination
• Acting as an indicator or warning

Making

A PCB-mounting terminal block

Normally a bulb would not be mounted on the PCB. Usually a terminal block is mounted on the PCB and wires from this are connected to the bulb holder.

Build and test the unit that will provide the driving input signal before adding the bulb.

Testing

Make sure that the bulb switches on and off as power is applied from the driver unit.

Fault finding

If there is a fault, check the bulb by removing it from the circuit, placing it in a new holder and applying power to it directly. Check the voltage at the terminals of the terminal block. Check the resistance between the leads to the bulb holder (with the bulb inserted).

Alternatives

• LEDs are cheaper but they do not give as much light.
• Electro-luminescent displays can be used to produce more sophisticated visual displays but are more expensive.
• Low voltage halogen lighting can be controlled by electronics via a relay. It provides much brighter lighting than an ordinary filament bulb but is more expensive.

Web links

• Building blocks

Return to list of datasheets

Buzzer (and Piezo sounder) - Output Device

What does it do?

The buzzer subsystem produces an audible tone when powered.

How does it operate?

Possible applications

• Making a warning sound
• Signalling that something has happened

Making

Buzzers have a positive and a negative terminal, marked on their case. The positive terminal should be connected to the positive voltage supply. The negative terminal should be connected to the signal from the driver.

The graphic on the left shows how part of the PCB might look for a PCB-mounted buzzer connected to a driver.

How part of the PCB might look

If a buzzer with flying leads is used then a terminal block is mounted on the PCB and wires from this are connected to the buzzer.

Build and test the unit that will provide the driving input signal before adding the buzzer.

Testing

Make sure that the buzzer switches on and off as power is applied from the driver unit.

Fault finding

If there is a fault, check that the buzzer has been connected the right way round. Check the buzzer by applying power to it directly.

Alternatives

• Other devices, such as LEDs can provide a warning signal – LEDs are cheaper than buzzer but do not make a noise
• Piezo transducers can produce a range of tones (frequencies) but they need to be driven by an a.c. signal e.g. from a PIC.

Web links

• Building blocks

Return to list of datasheets

LED - Output Device

What does it do?

The LED (light-emitting diode) subsystem converts the input signal into light.

How does it operate?

LEDs are diodes that emit light when a current passes through them. The current enters the anode and leaves the diode at the cathode.

Some process units provide enough current to drive LEDs. Typical LEDs require currents in the range 2 – 25mA.

If CMOS ICs or a higher current LED are used then a driver (transistor, Darlington or MOSFET is needed to boost the current.

LED circuit Click on the circuit diagram to download a Livewire file of the circuit that you can investigate and add to your own circuit.	The circuit on the left shows the circuit needed with a driver. The LED, in series with a current-limiting resistor R1, is connected between the supply rail (+Vs) and the input signal from the driver. The LED and resistor act as a load on the driver. When the input signal coming into the LED subsystem is low, a potential difference across the LED causes current to flow. It is this current that causes the LED to glow.
LED circuit for use with higher current process units Click on the circuit diagram to download a Livewire file of the circuit that you can investigate and add to your own circuit.	PICs, 555 Timer ICs and the LM324 op-amp can provide higher currents and can drive some LEDs without needing a driver unit. Check the data for the LED and the process unit to make sure that the process unit can provide more current than is needed by the LED. If this is possible, the LED is connected to the 0V rail (as on the left) rather than to +Vs
	LEDs come in a variety of colours and sizes. Flashing LEDs are available. Infrared (IR) LEDs produce infrared radiation which we cannot see with our eyes, but which is used in infrared communication systems. The infrared can be ‘seen’ with a digital camera.
	Rainbow LEDs e.g the Rapid 55-1904, cycle through the colours red, green, blue and white for a few seconds each.
	'Full colour' LEDs e.g. the Rapid 56-0672 contain a red, green and blue LED (the primary colours for light) and so, by turning different LEDs on and off, the full spectrum of colours can be displayed.
The series resistor R1 is used to limit the current passing through the LED. Using Ohm's law the value of this current-limiting resistor can be calculated: The voltage Vf across the diode is typically about 2V and the forward current If is about 10mA = 0.01A. So, for a 5V power supply: The nearest preferred (E12) value would be 330R.

Brightness
The ‘brightness’ (scientifically this is called the ‘luminous intensity’) of a LED, as perceived by the human eye, is expressed in candelas or milli-candelas (mcd). A low cost LED might have a luminous intensity of 10 – 100mcd. High brightness LEDs can have luminous intensities of 10,000 – 100,000mcd, which is as bright as a 60W light bulb.

The ‘viewing angle’ of a LED expresses how sharply concentrated the beam is. A narrow viewing angle is suitable for illumination but a wide viewing angle is needed for a LED being used as an indicator.

Possible applications

• Acting as a signal
• Providing illumination
• Decorative or mood lighting

Making

	A LED must be connected the right way round, with its anode positive and its cathode negative. Otherwise it will not work, and it might be damaged. There are two ways to spot the cathode. Usually one side of the LED is flat, and this on the same side as the cathode. The cathode leg is also shorter than the anode leg.
How part of the PCB might look	Normally a LED would be mounted on the PCB, as shown on the example on the left.
	It is also possible to mount LEDs in the product case in a panel clip as on the left. In this case a terminal block is mounted on the PCB and wires from this are connected to the LED.

Build and test the unit that will provide the driving input signal before adding the LED.

Testing

Make sure that the LED switches on and off as power is applied from the driver unit.

Fault finding

If there is a fault, check the voltage across the LED when it should be on (it should be about 2V). Check that the LED is the right way round. Check the value of the resistor.

Alternatives

• A bulb can provide more light but is larger and more expensive
• Electro-luminescent displays can be used to produce more sophisticated visual displays but are more expensive.

Web links

• Building blocks
• LED tutorial
• LEDs and electronic product design
• Producing different colours by mixing light
• Manufacturer’s data sheet for OSNR516A red LED
• Manufacturer's data sheet for full colour LED
• Manufacturer's data sheet for rainbow LED

Return to list of datasheets

Liquid crystal display (LCD) - Output Device

What does it do?

Liquid crystal displays are very useful for displaying both text and numbers. They display information sent from a PIC.

How does it operate?

	These notes refer to the Serial LCD/clock module available from Rapid (order code 13-1266) and Revolution (AXE033). This is one of the simplest and lowest cost LCDs available. The LCD comes in kit form with a pre-populated PCB. Construction only involves connecting the LCD to the PCB.
	Sending data from the PIC to the LCD module involves sending what is called ‘serial data’. This means that the bits that represent a number or a letter are sent one bit at a time. It is not necessary to understand a lot about serial communication to use an LCD, but details can be found on the web.

The details of how to send data from the PIC to the LCD depend on the PIC programming software. The software used must include a command to send serial data.

For example, in the case of PIC Logicator the command SerOut is used.

The LCD module has a number of special control commands (full details are in supplier’s data sheet). Common control commands are:

• 254,1 Clear Display (must be followed by a delay of at least 30ms)
• 254,128 Move to line 1, position 1

Possible applications

• Display for a weather station
• A toy for a child that aids reading or counting
• A multi-function clock
• A metronome that shows beats per minute and the musical name of the tempo

Making

There are a number of connection pads at the side of the LCD PCB. The only ones that are needed for simple applications are those labelled ‘V+’, ‘IN’ and ‘0V’.

‘V+’ and ‘0V’ are connected across the power supply. 5V to 6V is suitable. For a 4.5V power supply, a wire link needs to be added (see data sheet for details).

The ‘IN’ terminal is connected to the serial data signal from the PIC.

Full details of all the connection are given in the data sheet.

Connections to the LCD

Testing

Connect a power supply to the main connection header. The LCD should display the message ‘00/00/00 00:00’ when the two CLK contacts are shorted and once the contrast has been adjusted (via the variable resistor marked ‘contrast’).

Once the ‘00/00/00 00:00’ test message is displayed, write a short program to send data from the PIC e.g.

• Delay for 0.5s to allow the display to stabilize
• Clear the display
• Delay for 30ms
• Send a message e.g. ‘Hello’ to the display

Fault finding

If the LCD does not display a message check the power, the contrast and the 14 connector pins.

Alternatives

• 7-segment displays can display a few digits. They are less expensive but can only display a very limited range of characters
• Dot matrix displays can display numbers and characters, but they are more complicated to use
• Electroluminescent displays can be used to illuminate a fixed message, but the message cannot be changed
• The LCD described can be extended by including an optional clock for applications where accurate and/or long timing is needed

Web links

• Supplier’s data sheet for LCD
• How LCDs work

Loudspeaker - Output Device

What does it do?

Loudspeakers are used to produce sounds.

How does it operate?

Possible applications

• Playing tunes with a PIC
• Part of a radio or other communication system

Making

Make sure that the subsystem providing the a.c. signal voltage to the loudspeaker is working correctly before adding the loudspeaker.

Loudspeakers with flying leads can be connected to the PCB using a terminal block.

A PCB-mounting terminal block

In the case of a PCB-mounted loudspeaker the position of the pads on the PCB needs to be adjusted to fit the pin spacing of the loudspeaker and allowance needs to be made for the size of the loudspeaker.

Testing

Send signal voltages of various frequencies to the loudspeaker and check that it responds.

Fault finding

If there is a fault, check that an a.c. signal voltage is coming into the loudspeaker.

Alternatives

• Piezo transducers do a very similar task. The uncased piezo transducer is cheaper than the PCB-mounted loudspeaker, but it only produces very quiet sounds. Most piezo transducers are slightly smaller than the PCB-mounted loudspeaker, but they are more expensive and produce poorer sound quality.
• Buzzers and piezo sounders can be used to produce a single tone. They are simple to use and require a d.c. signal voltage.

Web links

• Building blocks
• Manufacturer’s data sheet for HSM23A-8 PCB-mounted loudspeaker (Rapid Order code 35-3540) and a typical small case-mounted loudspeaker (Rapid Order code 35-0380)

Motor - Output Device

What does it do?

The motor subsystem provides rotational motion when powered.

How does it operate?

Possible applications

• Causing movement. Using mechanisms, the rotational movement from the motor can be converted to linear or reciprocating movement.

Making

Normally a motor would not be mounted on the PCB. Usually a terminal block is mounted on the PCB and wires from this are connected to the motor.

The direction of rotation of the motor is reversed if the terminal connections are reversed.

A PCB-mounting terminal

Small electrical motors rotate at high speed. It is usually necessary to use them with a speed-reducing gearbox.

Build and test the unit that will provide the driving input signal before adding the motor.

Testing

Make sure that the motor turns when power is applied from the driver unit.

Fault finding

If there is a fault, check the motor by removing it from the circuit and applying power to it directly. Check the voltage at the terminals of the terminal block.

Alternatives

• Stepper motors and servo motors can be used to produce rotation through a precise angle
• Solenoids produce linear movement. The force produced by electronic solenoids is weak.
• Solenoid valves used to activate pneumatic cylinders produce more powerful linear movement but they are expensive and need a supply of compressed air

Web links

• Building blocks

Return to list of datasheets

Piezo transducer - Output Device

What does it do?

Piezo transducers are used to produce sounds.

How does it operate?

Possible applications

• Playing tunes with a PIC
• Part of a radio or other communication system

Making

Make sure that the subsystem providing the a.c. signal voltage to the piezo transducer is working correctly before adding the piezo transducer.

Piezo transducers with flying leads can be connected to the PCB using a terminal block.

Make sure that the piezo transducer is connected the right way round.

A PCB-mounting terminal block

In the case of PCB-mounted piezo transducers the position of the pads on the PCB needs to be adjusted to fit the pin spacing of the transducer and allowance needs to be made for the size of the transducer.

Testing

Send signal voltages of various frequencies to the piezo sounder and check that it responds.

Fault finding

If there is a fault, check that an a.c. signal voltage is coming into the piezo transducer. Check that the piezo transducer is connected the right way round.

Alternatives

• Small PCB-mounted loudspeakers do a very similar task. They are cheaper than most piezo transducers (apart from the uncased version) and produce a better sound quality. They are slightly larger than most piezo transducers.
• Buzzers and piezo sounders can be used to produce a single tone. They are simple to use and require a d.c. signal voltage.

Web links

• Building blocks
• Manufacturer’s data sheet for uncased piezo transducer and piezo transducer with flying leads

Servo motor - Output Device

What does it do?

Servo motors turn through a precise angle. They are controlled by a series of pulses. The width of the pulses to the servo motor controls the angle through which it turns. It is easy to use them with PICs (if suitable programming software is used).

How does it operate?

Possible applications

• Moving a barrier or the arm of a robot to different controlled positions
• Moving an item on a point-of-sale display through various angles
• Servo motors are widely used in radio control

Making

A three pin PCB-mounting terminal block	How part of the PCB might look
The three wires to the servo motor can be connected to the PCB using a three pin PCB-mounting terminal block.	Build and test the unit that will provide the input signal before adding the servo motor.

Testing

Before connecting the servo motor, use a multimeter to check that the terminal on the connector that will be connected to the:

• red lead to the servo motor is at +5V;
• black lead to the servo motor is at 0V.

Connect the servo motor. Write a short program that will:

• produce 0.75ms pulses
• delay for a few seconds
• produce 2.25ms pulses
• delay for a few seconds
• repeat this sequence

Fault finding

If there is a fault, check the three wires to the servo motor are connected correctly and are at the correct voltage.

Check that a suitable stream of pulses is being fed to the servo motor by using an oscilloscope.

Alternatives

• A stepper motor moves through a fixed angle. Stepper motors are less expensive than servo motors, but they cannot be moved as precisely and the PIC programming involved is more complicated.
• A solenoid can be used to move an object between two fixed positions. Solenoids are less expensive than servo motors, but they can only move to two positions and they are not as powerful.
• A d.c. motor can be used to move an object between two fixed positions, provided two microswitches are used to stop the motor when it gets to the required position. d.c. motors are less expensive than servo motors, but they can only move to two positions and the programming (or electronic hardware) involved is more complicated.

Web links

• Manufacturer’s data sheet for Sanwa type SW102Z servo motor
• An alternative servo motor (order code AXE 030) is available from Revolution Education
• What’s a servo?

Solenoid -Output Device

What does it do?

The solenoid subsystem provides linear motion.

How does it operate?

Possible applications

Solenoids only provide quite a weak force and a short movement.

• Opening and closing a small door
• Raising and lowering a lightweight barrier

Making

Normally a solenoid would not be mounted on the PCB. (PCB-mounted solenoids are available, but they are more difficult to link to the other mechanical parts.) Usually a terminal block is mounted on the PCB and wires from this are connected to the solenoid.

A PCB-mounting terminal block

Build and test the unit that will provide the driving input signal before adding the solenoid.

Testing

Make sure that the solenoid moves in and out as power is applied from the driver unit.

Fault finding

If there is a fault, check the solenoid by removing it from the circuit and applying power to it directly. Check the voltage at the terminals of the terminal block.

Alternatives

• d.c. motors, stepper motors and servo motors can be used to produce rotation through a precise angle. A suitable mechanism, such as a rack and pinion, can convert this to linear movement. The stepper and servo motors are more precise, but more expensive.
• Solenoid valves used to activate pneumatic cylinders produce more powerful linear movement but they are expensive and need a supply of compressed air
• 'Smart wire' is a shape memory alloy (SMA) that changes its length when a small current is passed through it. It is cheaper than a solenoid but the force and movement produced are small.

Web links

• Building blocks
• Manufacturer's data sheet for Rapid 60-3205 6V PCB-mounted solenoid
• Manufacturer's data sheet for Rapid 60-3240 12V latching solenoid

Return to list of datasheets

Sound and Music - Output Device

What does it do?

Units are available that will play a variety of tunes and interesting sounds and that will record and replay sounds.

How do they operate?

A variety of modules are available that can be used to add sound or music to electronic systems.

The simplest and lowest in cost is the Melody generator IC which plays ‘It’s a small world’.

Two Melody ICs are available, each of which gives a choice of three tunes.

A 20 second sound recorder module is available that allows up to 20 seconds of sound to be recorded and replayed.

Voice modules make a variety of sounds.

All of these modules can be operated on their own; or they can be triggered by a transistor used as a switch, so that they become output subsystems.

The Melody generator IC (Rapid 82-0044) plays the tune ‘It’s a small world’ when a supply voltage is connected across it. The supply voltage should be in the range 1.3 – 3.3V. If a higher voltage supply is more convenient then a resistor can be included in series between +Vs and pin 2. The current consumed is about 2mA, so a +5V supply would require a series resistor of about 1k. Pin 1 is connected to a piezo transducer  or a small loudspeaker.

Melody ICs (Rapid 82-0074 and 82-0076) can play three alternative tunes. The 82-0074 can play: Japanese lullaby Brahms lullaby Rock-a-by-baby The 82-0076 can play: Jingle bells We wish you a merry Christmas Santa Claus is coming to town

When the push switch labelled ‘Tune 1, 2 or 3’ is pushed the corresponding tune plays. It keeps repeating if the switch is held down. If the switch labelled ‘Cycle’ is pressed then the IC plays Tune 1, then Tune 2 and then Tune 3.

The supply voltage should be in the range 3.0 – 3.5V. If a higher voltage supply is used then a resistor can be included in series between +Vs and pin 7. The current consumed is about 10mA, so a +5V supply would require a series resistor of about 220W.

Pins 9 and 10 are connected to a piezo transducer or a small loudspeaker.

20 second sound recorder module

The 20 second sound recorder module (Rapid 13-0660) records up to 20 seconds of sound when the white 'Record' button is pressed. The LED lights while recording is taking place. The recorded sound can be replayed by pressing the green 'Play' button. The recorded sound is not lost when the batteries are changed.

The supply voltage needs to be in the range of 4.5 - 6.0V.

Voice modules are designed for use in children’s toys. When the top is pushed down they make a variety of sounds: ‘witch laughter’, ‘lion roar’, ‘owl hoot’, ‘bird song’, ‘snoring’ and one saying ‘I love you’.

Including sound and music in a larger system

Transistor switch circuit to control sound and music units

All of the above sound and music units can be incorporated into larger systems as output subsystems. This can be done by using a transistor switch (with its input signal coming from the rest of the electronic system) to turn the sound and music unit on and off. Any low current transistor is suitable.

In the case of the Melody generator IC and the Melody ICs the transistor switch subsystem is used to switch current to the unit on and off by placing the transistor switch in the circuits for the units in place of the wire link shown in red in the above circuit diagrams. The emitter (‘E’) terminal of the transistor switch should be connected to the 0V line. The sound unit will operate when the signal into the transistor switch is high.

In the case of the Melody ICs any of the switches marked ‘Tune 1, 2 and 3’ and ‘Cycle’ can also be replaced with a transistor switch.

The 20 second sound recorder module and the voice module can be operated with a transistor switch by connecting the emitter (‘E’) terminal of the transistor switch to the pad marked ‘A’ and the collector (‘C’) terminal of the transistor switch to the pad marked ‘B’. Alternatively the on/off switch can be shorted out, the batteries removed and replaced with a power signal from the main subsystem – just like any other output device.

Some music generators cannot be easily adapted in this way. For example, the terminals of the Rapid Music Unit (Order code 13-0620) are made from a metal that cannot be soldered to.

Possible applications

• Novelty items
• Children's toys
• Timers

Making

If the sound unit is being controlled by a transistor switch, a PCB-mounted terminal block is useful for connecting the wires between the electronic control system and the sound unit.

A PCB-mounting terminal block

Testing

First check the operation of the sound unit without any connections. Next check the operation using the transistor switch. This can be operated by connecting a piece of wire to make its input signam high or low.

Fault Finding

If there is a fault, check the orientation and pin connections of the transistor. Check the voltage of the input signal to the transistor.

Alternatives

• A piezo transducer or a small loudspeaker can be used to produce sounds with a PIC or pulse generator. However these systems only produce musical notes.
• A buzzer can be used to produce a simple warning sound.

Web links

• Manufacturer's data sheet for Melody generator IC
• Manufacturer's data sheet for Melody ICs
• Manufacturer's data sheet for 20 second sound recorder module

Return to list of datasheets

Stepper motor - Output Device

What does it do?

A stepper motor turns through precise steps. So it is useful for moving things through an exact angle or distance.

How does it operate?

To drive the motor forward by one step (7.5^o) the following sequence of signals needs to be applied to the coil connections:

To drive the motor backwards by one step the sequence is:

Possible applications

• Moving a lever a precise amount
• Raising and lowering a sign a fixed distance

Making

Cut off the connecting plug that is attached to the stepper motor.

The four wires to the stepper motor can be connected to the PCB using a four pin PCB-mounting terminal block.

Build and test the L293D that will provide the input signals before adding the stepper motor.

A four pin PCB-mounting terminal block

Testing

Before connecting the stepper motor, use a multimeter to check that the four signals from the L293D driver are correctly connected to the terminals on the 4-pin terminal block.

Connect the stepper motor. Write a short program that will:

• move the stepper motor a few steps forward;
• move it a few steps in reverse;
• repeat this sequence.

Fault finding

If there is a fault, check that the four wires to the stepper motor are connected correctly.

Alternatives

• A servo motor can be used to turn an object through a precise angle. It is more accurate than a stepper motor but it is more expensive and requires a separate power supply.
• A solenoid can be used to move an object between two fixed positions. Solenoids are less expensive than servo motors, but they can only move to two positions and they are not as powerful.
• A d.c. motor can be used to move an object between two fixed positions, provided two microswitches are used to stop the motor when it gets to the required position. d.c. motors are less expensive than servo motors, but they can only move to two positions and the programming (or electronic hardware) involved is more complicated.

Web links

• Building blocks
• Manufacturer’s data sheet for SM 46 stepper motor
• Stepper motor types and details

Alternative Power Sources - Power Supply Unit

What does it do?

Non-rechargeable batteries are widely used but they cause environmental damage. Alternatives include: USB power from computers, solar power, hand cranked power, recycled old mobile phone chargers and fuel cells. Rechargeable batteries and supercapacitors are also more environmentally friendly and are described in separate data sheets.

What’s the Problem?

People in the UK use about 680 million batteries every year (about 21 batteries per household), producing 20-30,000 tonnes of domestic waste. Batteries can contain highly toxic heavy metals such as cadmium and mercury. When the battery casings corrode these heavy metals leak into the ground and contribute to soil and water pollution and endanger wildlife. Cadmium, for example is toxic to aquatic invertebrates and can bio-accumulate in fish. Electronics is the area in Design, Engineering & Technology (and in education overall) that makes the greatest use of disposable batteries and we should be actively looking for alternatives.

What are the Alternatives?

Many people have access to a computer with a USB socket. This can be used to power electronic systems and to refresh rechargeable batteries and supercapacitors. Solar panels convert sunlight to electrical energy. A hand crank can power a dynamo to produce electrical energy. Fuel cells are relatively expensive. They use hydrogen as a fuel to produce electricity.

The notes below explain how each of these alternatives to disposable batteries can be used.

USB

The USB port of a computer provides a reliable 5V supply able to source 100mA. This is sufficient to power small electronic systems or to recharge batteries or supercapacitors. The connections for series ‘A’ plugs (generally used at the computer end of a USB wire) and sockets are:

Connections on the plug at the end of a USB cable

Connections looking into the plug at the end of a USB cable and the device socket on the computer

Pin	Name	Description
1	VBUS	+5 VDC
2	D-	Data -
3	D+	Data +
4	GND	Ground(0V)

From the table above, as you look at a USB port on a computer (assuming the visible metal connectors are underneath (as shown above right) the left-hand connector is +5V and the right-hand connector is GND (0V). As we are simply using the USB port as a power source, we can ignore the data lines (connectors 2 and 3).

Veroboard cut to fit into a USB socket	PCB designed to fit into a USB socket, viewed from the component side

None of the major suppliers of components to schools seems to stock the cable end USB connectors. However, the 0.1” spacing between the tracks on Veroboard (stripboard) is the same as the spacing between the connectors on the USB socket – and Veroboard is just the right depth to slot into the connector. The same result can be achieved using tracks of the correct width and spacing (0.3”) on a PCB.

The USB port can supply up to 100mA and is protected against short circuits.

Solar Power

Solar cells convert light energy to electrical energy. They can be used to power small electronic systems or to refresh rechargeable batteries or supercapacitors.

The Rapid 42-0240 produces an output of up to 3V and 100mA.

The MUTR EL1 006 produces an output of up to 4.4V and 90mA.

The circuit on the left can be used to recharge a battery (or a supercapacitor) using a solar cell. The diode allows current to flow in only one direction. This prevents battery power discharging through the solar cell when light levels are low. There is a voltage drop of about 0.6V across the diode.

It is important to be aware of safety issues before recharging batteries.

To find the actual current output of a solar cell you need to experiment with a multimeter.

Hand-cranked Power

Hand-cranked power can be used to recharge commercial radios, torches, phones etc. However the core winding units to achieve this don’t seem to be available to education users.

An interesting approach using LEGO® components to create a charger for devices powered by USB is described at Instructables.com

LEGO® hand-crank – from Instructables

Recycled mobile ’phone chargers

A good alternative to batteries is old mobile ’phone chargers. The plug that fitted into the ’phone can be removed and replaced with a suitable plug to fit an appropriate socket on the circuit board.

Provided the power requirements of the circuit can be fulfilled these chargers can be a cheap or even free source.

Fuel cells

In a basic fuel cell hydrogen and oxygen are combined to create water; this is a chemical reaction that releases energy and the cell is designed to use this energy to separate charges creating a voltage of about 0.7V. The main difference between a fuel cell and a battery is that when the hydrogen in a fuel cell runs out the cell can be refuelled by adding more hydrogen (the oxygen used come from the air).

It is worth noting that the chemicals used fuel cells have to be made in some way – usually using electricity generated by some other (usually unsustainable) method. However fuel cells are useful for situations where electrical power is wanted away from mains electricity; in portable consumer goods and for transport. Increasingly you will find fuel cells replacing batteries in products such as mobile phones.

There appear to be few options currently available for education to explore fuel cell technology.

Rapid (06-6520) fuel cell car kit. Note that this requires a hydrogen fuel source.

Economatics have a ‘Solar Hydrogen Science Kit’ that includes a fuel cell.

Web links

• Battery waste and recycling
• Electronics and the environment
• USB connectors
• Rapid 42-0240 solar panel data sheet
• Fuel cell primer video
• How fuel cells work
• Low temperature Polymer Electrolyte Membrane (PEM) fuel cells

Return to list of datasheets

Battery - Power Supply Unit

What does it do?

Batteries are used to provide electrical power to portable electronic systems.

How does it operate?

In most electronic systems, nearly all the power from the battery is used by the output device(s). The first task in selecting a battery is therefore to investigate the output devices needed and to find:

the voltage they need (the battery will provide this voltage);
the current consumed (this will directly influence the battery’s useful life, depending on the battery ‘capacity’ – see below).

An ‘ideal’ battery provides a steady voltage, and as much current as is needed. In practice, the voltage available from a battery falls as it is discharged and as the current drawn increases.

Environmental Issues

The average UK household uses 21 batteries a year. On average, only one of these is recycled. The others go into landfill sites. Much of this waste could be eliminated by using rechargeable batteries.

Some battery types contain cadmium or mercury. These chemicals cause environmental damage and batteries that contain them should not be disposed of in a rubbish bin. Consult your local waste disposal service.

Key features of a battery are:

Type
The battery ‘type’ refers to the materials that are used to make the battery. Common battery types include: zinc chloride, alkaline, nickel cadmium, nickel hydride, lithium polymer, lithium iron sulphide, lead acid, silver oxide and lithium manganese.

Case
Common cases include AAA, AA, C, D, PP3, coin cell and button cell. The type of case provides details of the shape and dimensions of the battery.

Voltage
Manufacturers and suppliers usually provide information about the voltage that a battery provides when it is fully charged, and not supplying current.

In practice, the voltage a battery provides falls as it is discharged by providing current to the electronic system. Some manufacturers provide graphs of typical discharge characteristics in data sheets.

In addition, the voltage a battery actually provides when a current is drawn is less than its voltage when it is not supplying a current. This is due to the battery’s ‘internal resistance’ (see below).

Capacity
The capacity of a battery indicates how much current it can provide over a period of time. Capacity is usually specified in milli Amp hours (mAh) – the current provided multiplied by the time to discharge. For example, if a battery has a capacity of 2000 mAh, it could provide a current of 2000mA for one hour before it was exhausted, or 1000mA for two hours, or 100mA for 20 hours.

The capacity of a battery depends on the battery type (some types last longer than others) and on its case (generally speaking, the bigger the battery, the higher its capacity).

The tables below show battery capacities for a range of battery types and cases.

Internal Resistance
A practical battery behaves as if it were made up of an ‘ideal’ battery (providing a constant ‘ideal’ voltage VI) in series with a resistor (called the battery’s internal resistance – RI) inside the practical battery.

In consequence, when a battery provides a current (IB) the voltage across the battery terminals (VB) is VI minus the voltage dropped across the internal resistance (IB RI). So:

The most important point to bear in mind about the internal resistance of a battery is that the battery type should be chosen so that its internal resistance is well below the resistance of the load. Otherwise most of the battery’s energy will be wasted as heat in the battery’s internal resistance – not in providing energy to the load.

The internal resistance depends on the battery type and case and generally increases as the battery discharges. The table below shows typical values for a range of battery types and cases.

Details of Battery Types
Probably the battery case most widely used in schools is the ‘AA’. The table below gives details of common AA battery types.

The data given for the ‘Initial voltage’ (the voltage of a ‘fresh’ battery) is taken from manufacturers’ data sheets. Some manufacturers’ data sheets give the battery Capacity directly. In some cases the Capacity has been estimated from graphs of the battery discharge characteristics given in data sheets.

Data on internal resistance is not always quoted by manufacturers. In those cases the estimated figure quoted is based on measurements made on one battery sample. The figures given for internal resistance are for a fresh battery. The internal resistance increases as the battery discharges.

Rechargeable batteries
In principle, rechargeable batteries can help to reduce waste and pollution. However, there are potential problems in using some types of rechargeable batteries in school electronics. In particular, because the internal resistance of nickel hydride and nickel cadmium batteries is much lower than non-rechargeable types, they deliver a much larger current if a short circuit is accidentally applied across them. If this happens, the battery can overheat and possibly explode. This can present a health hazard and can also damage the battery.

If the battery can be completely enclosed (to prevent short circuits) and a voltage regulator (hotlink to data sheet) can be included inside the enclosure, then the voltage regulator will prevent hazards and damage to the battery because it includes current limiting circuitry.

Safe battery recharging
There are important safety limits to the current and voltage that can be used to recharge batteries. Commercial battery rechargers work within these limits. However, if you are planning to build your own recharger, these limits need to be observed.

Most modern batteries can be charged at quite a high current. For example you could charge a 2,000mAhrs battery with a 500mA current for just over 4hrs and it would be fully charged. However, if you keep on charging it beyond that 4hrs you could seriously damage the battery (or even cause an explosion). NiMH batteries have a protective mechanism when they get overcharged and attempt to dissipate the excess current as heat. However they can usually only managed to discharge one tenth of their total current capacity as heat. What this means in practical terms is that if you charge a 1,300mAhrs battery with 130mA then it will survive, but get warm if you overcharge it for a while. However if you overcharge it with a 500mA current the risk of explosion occurs.

Cases

The information above applies to AA case batteries – which are widely used in schools. There are situations where other battery sizes are useful, and many of the battery types described above are available in a range of cases.

To estimate the capacity of a particular type of battery in the relevant case size from the table below, multiply the capacity of the AA battery of the same type (listed above) by the ‘Case Capacity Factor’. For example, if we wanted to find the capacity of a Power One alkaline C battery:

• the capacity of a Power One alkaline AA battery is 2,600mAhrs;
• the Case Capacity Factor for a C battery is 2.9;
• so the capacity of a Power One alkaline C battery should be 2,600 ´ 2.9, which is about 7,550mAhrs.