This is an Automatic Cooling Fan Controller circuit. This circuit is very useful during summer to keep our important item and rigs such as VHF and UHF transceivers staying cool. Because VHF and UHF transceivers were sold with no cooling fans and oversized heat sinks. Here is the circuit:

Automatic Cooling Fan Controller circuit schematic
This circuit uses The thermistor, R1. It is the heart of this circuit. The resistance of the thermistor is about 10,000 ohms at 70 F. The resistance will decrease, As the thermistor heats up, until the circuit reaches voltage threshold determined by R4. The transistor Q1 is saturated due to the op amp output changing from high to low. So the current will flow through the emitter to the collector.To reduce the noise of the fan this circuit use Capacitor C1. It also provides capacitive start for the motor. If fan noise becomes audible in rig’s receiver, we should increase the value of C1. We can replace R4 with a 10K trimmer pot to change temperature when the fan to turn on. [Source: Radio Amateur Society of Norwich]