The controller uses one or more ordinary silicon diodes as a sensor, and uses a cheap opamp as the amplifier. This circuit designed to be use 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.
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