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 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.
OS DIODOS SENSORES SÃO DIODOS RETIFICADORES COMUNS SILICIO,A TEMPERATURA DE DISPARO É REGULADA EM VR1 TRIMPOT, SE QUISER USE DISSIPADOR EM BD140