As part of my continuing obsession with coffee I discovered that I could get a lot cheaper specialty coffee, roasted closer to my personal taste, from the regions I prefer (East Africa), by just roasting myself. Sweet Maria's provides troves of information about how to home roast, and one of the suggestions they make for first timers is to use an inexpensive popcorn popper. They recommend the Nostalgia.
So I bought it, and was dissatisfied with the lack of control I had. I found out that there are a not insignificant number of home roasters who tried hooking up a PID controller to their popcorn popper (or generally modding it), and I found this decent write up on how to use an Arduino to hook up the heating element to a 40A relay and use a thermocouple with an RTU Modbus implementation to communicate with Artisan Scope, a professional grade open source program written in Python for monitoring and controlling roast parameters (more meant for large machines, but...)
There are some problems with Lukas' write up. Namely, that Artisan's internal PID system is really hard to get working. I ended up building in an internal PID using an Arduino library (you can see my code here). So Artisan just sends the PID values and the SV and monitors the temperature. I also found it unnecessary to use a Mosfet between the Arduino and the relay. And for debugging, I added an LCD screen (which is how I discovered that Artisan's PID was never working in the first place).
This project was insane fun. As a programmer, I often get a kick out of some piece of software working. But making something do stuff in meatspace is a rush I'd never experienced before. It was also the first time I'd done real soldering. It was a lot easier than I expected.