| Linear temperature sensors are used to monitor temperature accurately. They are particularly useful with PICs which have an analogue to digital converter. |
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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.
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Simple linear temperature 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. |
The supply voltage +Vs needs to be in the range 4V to 30V.
The output signal voltage (between pins 1 and 2) is 0V when the temperature is 0oC (the freezing temperature of water). For every 1oC increase in temperature, the output signal voltage increases by 10mV. So, at 25oC, the output signal voltage is 25 x10mV = 250mV = 0.25V. At 100oC (the boiling point of water) the output signal voltage is 1,000mV = 1.00V.
The LM35DZ is not available as a component in circuit simulation software popular in schools. In the circuit diagram on the left it is represented by a 3-pin single-in-line socket, which has the same ‘footprint’ on the PCB.
To produce a variable voltage for use in circuit simulations a potentiometer could be used. |
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Linear temperature sensor circuit for temperatures below 0oC |
In the simple linear temperature sensor circuit shown above, the LM35DZ can be used to monitor temperatures down to 0oC.
For temperatures below 0oC the circuit on the left (to which a diode has been added) can be used.
The voltage across a forward-biased diode is 0.7V = 700mV.
If the temperature is 0oC, the voltage between pins 1 and 2 of the LM35DZ will be 0V, but the voltage across the diode will be 700mV, so the voltage between pin 2 of the LM35DZ (the output signal) and the 0V line will be 700mV. |
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If the temperature is 100oC, the voltage between pins 1 and 2 of the LM35DZ will be 1,000mV, the voltage across the diode will still be 700mV, so the output signal voltage will be 1000mV + 700mV = 1,700mV = 1.70V.
If the temperature is -20oC, the voltage between pins 1 and 2 of the LM35DZ will be
-20 x 10mV = -200mV, the voltage across the diode will be 700mV, so the output signal voltage will be -200mV + 700mV = 500mV = 0.50V.
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The LM35DZ is accurate to at least 2oC.
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 = 36oC.
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 = -34oC.
Possible applications
Making
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Pins of the LM35DZ
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How part of the PCB for the simple linear temperature sensor circuit might look
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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
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