Geoff designed this USB PC case fan controller. It is used to control the speed of your fans depending on the temperatures in your case. Software that was developed for this project allows you to customize the temperature profiles for your computer.
The project is based on the PIC18F2550 that is connected to the computer via the USB and uses the standard Molex 4pin connector to access computer’s power supply. It has 4 analog inputs for temp sensors, and can control up to 8 fans.
One thing to note is that all the fan outputs work with 3 pin fans, while two are universal and work even with 4 pin PWM versions. The 3 pin fans are driven with a buck convertor. The UDN2981 provides the high side switch and diode that are driven from PIC’s PWM signals. A 100uH inductor and a 479uF capacitor complete the buck topology, thus providing variable analog output for the 3 pin fans.
Intelligent Fan Controller - [Link]