This circuit is designed to control small cooling fan, speed of the fan is controlled by the NTC thermisters these kind of small fans are used to reduce the temperature of semiconductor devices by the varying temperature speed of fan varies automatically.
Circuit diagram for fan speed control
Construction and Working
In this circuit power supply to the cooling fan given through four NTC thermisters. Each one have 470Ω resistance when the temperature increase this NTC gices low resistance to the power supply hence the fan speed increase by the way the speed of fan controlled automatically depends on temperature.