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!
Step 1: Parts
1. PCB. I pay by the square inch to have boards made so I try to keep them small. As such, this one is double-sided and uses plated-through holes. You're welcome to try to make one at home but I really recommend getting one professionally fabricated. I had mine made by BatchPCB, a division of SparkFun. You can buy a board at http://www.batchpcb.com/product_info.php?products_id=44036&check=c260fda1c9d3f5c8cb643726708f6a35 or download the Eagle files at the end of this Instructable and sort it out yourself.
2. Picaxe 18X. This has since been superceded by the Picaxe 18M2, which should be pin-for-pin compatible. (and slightly cheaper)
3. Temp Sensor. The picaxe can natively interface with a DS18B20 digital temperature sensor from Dallas/Maxim, which makes it super easy to write the code. I found this waterproof version on eBay, whick is far more suitable for immersion in an ethanol-water-sugar solution. Depending on what you're actually using you thermostat to control, you may be able to use the bare sensor, which looks like a TO-92 transistor. However the waterproof version should only set you back about $6. Note that the one I bought had the pins swapped, so that pin 1 of the molex plug was actually pin 3 of the sensor! It is possible to carefully remove the pins from the plug body and move them into their correct places, or you could attach the socket to the board the other way around. Whatever you choose to do, CHECK THE PINOUTS OF THE TEMP SENSOR! On the sensor I used, pin 1 [ground] was the bare shield wire, pin 2 [DQ] was white, and pin 3 [VDD] was red. I repeat, please check before you assemble the circuit!
4. IC sockets and pin headers. To raise the LCD module and the the up/down temperature switches off the board, so they poke out of the front panel. I used strips of header and pins, cut to size.
5. Resistors. I used high-brightness LEDs for the indicators, but used very high value resistors to keep the brightness down. If you want brighter LEDs you may need to use lower values for R8 and R9. Rememer that the green LED is driven from 5v from the picaxe, but the red LED gets 12v from the mosfet. Choose your resistors accordingly!