This simple temperature controlled DC fan circuit can be used to control any 12 volt 1A DC fan by activating it on a preset temperature. You can use this circuit in many electronic projects, which require a cooling fan. The circuit is very simple and using only five components.
The BD139 transistor is working as a switch here. When the 10K NTC thermistor will receive heat then its resistance will start decreasing and on a preset level the transistor BD139 will become switch ON and activates the 12 volt DC fan or motor. The 100K variable resistor is used to set the level of temperature or heat on which you want to switch ON the transistor.
The transistor will dissipate heat during operation therefore it is important to add some distance between the thermistor and transistor or the circuit will not work properly. Also use a suitable heatsink with the transistor.