heat increases for example you can use this circuit to activate a fan or use it as a temperature alarm circuit. These are only two examples where you can use this circuit but there are varieties of other applications where this circuit can be used.
The circuit is very simple to build and require only few easily available parts like a LM741 op-amp IC, thermistor, relay, a PNP transistor and some resistors. The activation of the relay switch on the required level of heat can be achieved by adjusting the 10K variable resistor. To achieve greater accuracy you can also experiment with other values of thermistor like 50K, 100K, 500K etc. and also the other values of variable resistor R1 like 50K, 100K or 500K etc.
A PNP transistor 2N4403 is working as a switch in the circuit you can also use other similar transistor if you not get this one. The circuit can be operated with 9 to 12V DC. If you are operating the circuit with 9 volts then make sure to use a 9 volt relay also.
The figure below shows a very useful and efficient project of a heat sensor circuit with LM741 IC. The circuit will activate a relay switch on the preset level of temperature or heat. This temperature sensor circuit can be used in many applications as a heat switch to activate any electronic device when