Check it out...
http://www.mtmscientific.com/tempkit.html
Its a small kit-built circuit that switches devices on and off based on temperature. An optional photosensor changes the input from temp to light...I.E....controlling fans when your lights are turned on.
Homebuilt chillers may be built from this device by incorporating an inexpensive mini-fridge and this device...A length of tubing leads from a pump in your sump into the fridge, coiling in a cold-water bucket and draining back into the sump. The temp sensor switches the pump on and off based on requirements. The temperature of the refridgerator is maintained by the fridge's onboard thermostat (normal operation).
The home-built circuit kit costs $30. The optional photosensor is an extra $5
The unit is similar to this one:
http://www.apogeekits.com/thermostat_vm137.htm
However, the thermostat from apogeekits.com is limited in range to a max of 86 degrees.
-Matt
