This project will show you how you can create your own programmable thermostat with temperature probe, an LED readout and ability to remote control outlets.
The core of this project is actually a driver for a 4 digit seven segment display.
There are lots uses for such a device. You can purchase something called a PID controller for about $120. The parts for this cost me about $66. If you can scrounge some of the parts you can do even better.
I use mine to control hot water baths for cooking meet. See my instructable on cooking perfect steak for why you might want this.
There are probably a dozen ways to do this. This was my way.
This project requires soldering small surface mount components (well one easy one IMHO). There are a lot of wires and it can be hard to debug certian problems. This is probably not a great project for a novice or the easily frustrated.
At this time the instructable isn't as detailed and foolproof as possible. My prototype works and this is my attempt to document the process. However, until I build my sweet new steampunk themed version, the instructable is going to be a little bit raw. You should be confident about these sorts of projects enough to do some minor debugging of the hardware and connections before you dive in. Just in case I got something wrong, you know. The software is solid at least. :)
Replace the $20 arduino in this project with $8 worth of electronics.
Generic Micro Controller switch outlets. I embedded this idea into this project. You can just build this stand alone version on hook it up to this project.
UPDATE: 2011-July-28 LM335A sensor version, cheaper, no surface mount soldering.
I am experimenting with replacing the thermocouple and booster with an LM335A sensor. It's not as accurate, and it won't go above 100 degrees C. However it's $1.50 compared to $28 for the thermocouple and booster. For souse-vide cooking it should work fine. There is a PCB123 diagram file in the diagram step if you want to see how to hook one up.
I have also gone to using the Atmega168 chip directly with the minimum hardware needed to support and program it. That's also in the PCB123 file along with a circuit board.
I plan to have circuit boards and solder your own kits available on Kickstarter soon. I am planning a run of 500. The kits will be ~$40 shipped. Let me know if you are interested. The Kickstarter never happened. I did another couple Arduino based Kickstarters and documented the projects on Instructables. Stay tuned, more to come I am sure.