John Carter made a nice AV fan controller for an audio/video cabinet. There is code and a diagram of the design on John’s site. This project uses the PIC AXE microcontroller, if you want to play with some of these chips I hear that Peter Anderson is a great resource with cheap shipping.

“This is a fan controller for an audio/video cabinet. It uses a PICAXE 08M and a DS18B20 temperature sensor. The fan is turned on at 30 degrees C (~86 F) and off at 28 degrees C (~82 F). The green LED is the activity indicator and also serves as a “ballpark” temperature indicator. All off-board connections are made via screw terminals. The left terminal block for the fan, the bottom block for power, the right terminal block for the DS18B20.”