Does anyone have plans for temperature controllers

	I've been searching and searching but I can't seem to find any plans for building
 a termperature controller for DIY undergravel heat cables.  I'd like to build one 
at the same time as I build the power supply.  I'm not looking
for anything fancy; just something that will turn the cables off
before overheating the tank and then turn them back on when it cools.
(ie. I don't need something that will be accurate to +-0.1 degrees.)
  I see lots of references (ex.  "[F] Modern Aquascaping- Part 9
(Final)" by David Hsu (4/27/93)) but no actual plans for a 
temperature controller that works.
		Also, are there any cheap controllers available commercially?
That would be the easiest thing for me since I'm not that 
electronically inclined.   

							Any help would be greatly appreciated,

																								Marc Chodos     choman at ucla_edu