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UPDATED:.20:42 02 January 2019

This circuit will operate a low voltage cooling fan when the temperature rises above a set point.

The sensor should not be placed directly in the path of the fan.

The temperature at which the fan will operate can be controlled by VR1 on the Comparator Switch.

Circuit Voltage: 6 to 12 volts - should be chosen to match the voltage of the fan.



Adjust VR1 with a screwdriver to the center position.
If the temperature you require is outside the range of VR1 on the Comparator Switch, then adjust VR1 on the Heat Sensor

The Schmitt setting should be selected by moving the jumper to the right position, creating hysterisis

As the temperature rises, the voltage from the Heat Sensor rises.

As the voltage rises above Vref on the Comparator Switch, the output turns on, switching the Darlington Driver on and the Fan.

As the temperature falls, the voltage from the sensor falls below Vref and the output turns off, switching the Darlington Driver off and the Fan.

There is some hysterisis in the switching meaning that the on temperature setting is slightly lower than the off setting. This prevents chatter (rapid switching) if the fan is too close to the sensor.


This version operates exactly the same as the first one except the Darlington Driver has been changed for a Relay. This means the fan voltage does not have to be the same as the supply voltage but you will need 2 supplies. If the fan voltage is between 6 to 12 volts then the circuit and the fan can use the same supply.

Take expert advice if you are using a mains voltage fan and ensure the current does not exceed 2 Amps or 450W of power!

  • Sensors in potential dividers.
  • Reversing compnents for opposite function.
  • Using a variable for R2 to create adjustment.
  • Operational amplifier as a comparator.

This is a common circuit for sensing heat, light and moisture and can be adapted for similar projects.

Vout = Vin x (R2 / R1+R2) to calculate voltages.

This can be expanded to using graphs showing resistance against temperature and predicting votages based on temperature in a potential divider.

Can be used with the Incubator Heater connected to a heating mat to create a controlled environment for seedlings or cuttings.

The Darlington Driver can be replaced with a Relay Blok to use a different voltage supply to the fan.

There are many applications for sensing heat and also by using other sensors instead of the thermistor, eg. LDR and moisture sensor.

Some examples are:

  • Soil Moisture Tester
  • Baby Bath Temperature Alert
  • Freezer/Fridge Alarm
  • Greenhouse Cooler.