Ever needed a temperature controlled cooling fan to only come on when necessary? Such as in an amplifier or in a power supply? Then this is the simple solution! This simple servo unit, based on a 555 IC with PWM (pulse-width modulation) to send burst of current to the fan motor to control the speed, uses an NTC (negative temperature coefficient) thermistor, which can be mounted in direct contact with the heatsink needed to be fan-assisted to help cool it down. The advantage is, it can be set to only come on when a certain temperature of the heatsink is reached (via RV1, which would be set through trial and error).

U1, an NE5534 opamp, is configured as a simple comparator, together with RV1 to set the temperature the fan will come on at. Once the resistance of TH1 decreases enough, due to temperature increase of the heatsink it's monitoring, and the compared voltage is higher than set by RV1; the comparator will switch high, increasing the control voltage on pin 5 of U2 (555 timer). The hotter TH1 gets, the higher U2's control voltage becomes. This, in turn, will change the duty cycle of U2's output on pin 3, increasing the speed of the fan.

Conversely, once TH1 begins to cool down, U2's control voltage decreases, increasing the duty cycle, thus decreasing the fan's speed. You can, with a bit of patience, adjust RV1 so the fan is spinning at low speed all the time, only to ramp-up to full speed once the heatsink it's monitoring gets hot enough. The frequency range of U2's output is around 1.2kHz (fan is not operating), to 211Hz (fan operating at full speed, minus the 0.6V drop of Q1).

Video of the completed project: