This circuit will turn on/off 12V DC fan or CPU fan when temperature above normal temperature.You can set turn on temperature by adjust VR1.
This circuit use an NTC (Negative temperature coefficient)which is a thermistor is one in which the zero-power resistance decreases with an increase in temperature. So If temperature increate the voltage at pin 3 on LM311 will decreated .The resistance of NTC is about 10K at 25'c.
VR1 should be multi-turn potentiometer type such 10K/25 turn.