What does it do?
| Turns something on, and then off, and keeps repeating this. |
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How does it operate?
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Low frequency pulses
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High frequency pulses
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A possible pulse unit circuit
Click on the circuit diagram to download a Livewire file of the pulse unit that you can investigate and add to your own circuit. |
The pulse subsystem 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 a variable resistor.
The pulse unit uses an oscillator consisting of three inverters along with an electrolytic capacitor and some resistors.
A variable resistor is used to control the pulse rate.
The time period can be changed by choosing different capacitors (C).
The time period for one pulse with the circuit on the left, when the variable resistor is on its maximum setting, can be calculated:
Time period = 0.044 ´ C
where C is the capacitance in mF.
For example, if C=22mF, the time period is about one second.
The pulse generated has a mark:space ratio of 1:1 meaning that the high and low parts of the pulse last for the same length of time. |
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
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