An Arduino is by no means expensive, but that still doesn't mean that it suits your needs right out of the box. If you want to make your own custom board, Make shows you how in a three-part series.

You also need to select the transistor by the type of fan that you use. In my case I used the well-known BD139 transistor and a 9V battery to provide power to the fan and transistor. The LM35 ...