temperture sensor

hi guys
i have a small assigment which i suppose to design a temperture controller. this controller have to be connected to a small fan to cool down the PC adapter.
i need little bit help, in my view i think i have to connect small sensor to a relay but HOW, with temperture range HELP.?!

Using a simple thermistor, you can make an on/off circuit using a comparator/amp, transistor, fan, and glue components. The thermistor is not linear, but it does not matter if you are just going to turn the fan on/off. If you are building something like a PID controller for the fan, you'll need some type of processor to control the motor speed.

Microchip has some great fan control chips Check out the TC642. Then all you need is a thermister and some resistors. The data sheet has a simple schematic.

Brent