I had (still have) a 24V fan. It's a Papst 8414M, rated 24V 1,4W. However, it says on it that it works for 12V-24V. Anyhow, it is a tornado at 24V, and at 12V it is quite nice. So if this fan can ...

The controller uses one or more ordinary silicon diodes as a sensor, and uses a cheap opamp as the amplifier. I designed this circuit to use 12V computer fans, as these are now very easy to get ...