An old CPU fan used to cool UNO board.  Automatically controlled by ambient room temperature through the use of a thermosistor and relay switch.

This is a very simple circuit and code. 

First circuit:

Connect one side of the 10k resistor to 5vDC, the other side goes to both UNO board pin A0 and the thermosistor.  The other side of the thermosistor goes to UNO board gnd. 


Second circuit:

Connect the relay, fan, and 12vDC (or whatever size power supply you need for your fan motor) and the UNO board together.

Relay pin 1 is connected to positive side of fan motor.  Relay pin 2 is connected to UNO D9.  Pin 3 is connected to positive side of 12vDC power supply. Pin 5 is connected to 5vDC.  Neg side of fan is connected to the Neg side of 12vDC.