I recently built a digital thermometer for one of my friends for his brewing. He prefers to keep things as manual as possible, but one thing bugs him: the thermometer is a delicate piece of equipment, and the mashing process requires careful monitoring of the temperature. Of course, digital thermometers come a dime a dozen, but how about a thermometer that can be programmed for a target temperature and tells him to increase or decrease the heat? It's a thermostat, yes, but for a DYI guy like him, a home-made one is better than anything you can find at a store.
Find the entire documentation here.