A temperature controlled fan circuit can be a very useful device to ON and OFF a DC 12V fan according to the changes in temperature. The circuit can be used in variety of electronic projects where cooling is required for example power supply circuits, audio amplifier circuits, battery chargers etc.
The circuit is built around a LM741 operational amplifier IC. The 10K NTC thermistor is used to sense the heat. The 10K variable resistor is used to set the circuit to activate the fan on your desired temperature. The output of the LM741 IC is only few milliamperes therefore we can not operate a fan directly with the IC due to which we have used a BD135 transistor which can handle upto 1A load.
The transistor will dissipate heat during operation, make sure to make some distance between the transistor and thermistor or the circuit will not work properly. Also use a suitable heat sink with the transistor.