Thermal Fan Controller, IC 741

This fan controller uses one or more ordinary silicon diodes as a sensor, and uses a cheap opamp as the amplifier. The circuit designed for 12V computer fans, as these are now very easy to get cheaply. These fans typically draw about 200mA when running, so a small power transistor will be fine as the switch. I used a BD140 (1A, 6.5W), but almost anything you have to hand will work just as well.

Circuit diagram:

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Thermal Fan Controller Circuit Diagram

Source: ESP