PWM, Fan controller
This circuit basically goes to PWM (Pulse Width Modulation). From this circuit i controlled a PC power supply fan. when the variable resistor is turned, the fan starts operating in a very slow speed and when the VR is turned more the speed of the fan increases. If you want you can just modify this circuit and use and temperature sensor and trigger the fan to cool the system. If you guys need the schematics for modifications, i can give it..