7 Temperature Relay Circuit

This project  is a simple temperature relay circuit that can be used as overheat protection system.
Initially,when the temperature in thermistor ( temperature dependent resistor) is low, the total resistance of thermistor and 150K resistor is lower than 100K. As a result, the voltage at noninverting input (marked  + ) is much greater than the inverting input ( marked - ). With this condition, the output of the comparator is high, thus  biasing the transistors 9013 and 2n3055 and opening the relay circuit .As a result, switching off the fan.
When the temperature reaches  maximum, the resistance of thermistor reaches in few mega-ohms and reversing the initial condition. as a result, turning on the fan.
Possible use of this circuit is overheat protection in any circuits or system. You can modify this simple temperature relay circuitry by simply adjusting the resistors depending on the desired temperature.
for more info and suggestions , just add comments.
Temperature Relay Circuit jpg


  1. X1 is a marking for jumper or not connected wire.

  2. and what is the part number of opamp?

  3. please, can you tell me the number of opamp or comparator, i dont know what is it, but i really need it for school project. thanks!

  4. You can use LM393, LM339, LM2901, or LM3302.

  5. How much the resistance of the thermistor should be? Is it concerned?

  6. You can use any thermistor (ranges from 300k to several Meg)