The temp controller is powered from the same 12v mains transformer that powers the heating pad, which is regulated down to 5v to drive the circuit. A picaxe 18X does the work, and a DS18B20 digital temp sensor is immersed in the brew to sense the temperature. The sensor is encapsulated in stainless steel so it can remain in the vat for the duration of the brewing.
The unit has a 2-line alphanumeric LCD to show the preset temperature, and the actual temp of the brew. Two buttons adjust the present temperature, and LEDs indicate when the heating element is on and/or the brew is at the desired temperature. A MOSFET switches the 12V at about 2 amps to drive the heating element. The mosfet I used can actually handle about 15 amps if required.
I used the thermostat to control a resistive heater which is sewn between a woollen blanket and cotton sheet, to form a thick wrap which I tie around the vat of home brew to keep it at a constant temperature for perfect fermentation, even during cold winter months in an unheated room. Thanks to Bec for sewing the heater element into the blanket!