The controller circuit can reduce a fan`s noise, power consumption, and wear, particularly in the presence of a low, fluctuating ambient temperature. Mount a temperature sensor in the fan`s airstream, and the circuit will adjust the fan speed as necessary to maintain a relatively constant sensor temperature. Input components Rl and Cl integrate the input square wave, producing a quasitriangular wave at the noninverting input of op amp IC1A. At this inverting input is a reference voltage that decreases as temperature increases.
Automatic-fan-speed-controller - schematic

The two-terminal sensor produces 1 p.A/0 K. The resultis a rectangular wave at the output of IClA with a duty cycle proportional to absolute temperature. Thus, a rise in temperature triggers a counteracting cooling effect by delivering more power to the fan. To calibrate the system with the sensor at room temperature, simply adjust R2 for a 50% duty cycle at Vi. The fan will switch off at approximately 0°C and will be fully on at 44°C.

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