Here's where I got the idea;
http://castboolits.gunloads.com/showthread.php?t=49410
The links on there give you where to go to buy the stuff, if you're interested that is.
Jector, I'm not sure how much the lee pot used to vary from on to off. I do have a Lyman thermometer, but the scale is hardly all that precise. By that I mean it's hard to tell with marks so close together that total 10 degrees/mark!
Jmorris, mine will "learn" all by itself, or if I tell it to, it'll pick it's own parameters. It's capable of maintaining +-1 degree. It'll also alarm if the temp goes 15 degrees above or below the set temp.