So I've been roasting coffee on and off for a few years. Below is a picture of my original method using an air-powered popcorn popper. I modified it such that the fan can be controlled independently of the heater. To make things simpler I experimented with different methods of temperature control. The picture below shows the Sparkfun electronics reflow oven controller connected to the popcorn machine. I programmed the reflow oven controller with a profile for roasting beans. It works well and allows for consistent hassle free roasting.
The big drawback of the hot air popper for a coffee roaster is the limit on the amount of beans you can roast at once. I rigged up a larger capacity roaster as seen in the pics below. It is essentially the pan and drive of a bread machine, an exhaust vent to take care of the chaff, a heat gun to provide the heat, a thermocouple and temperature controller to run the show. Results were good; consistent roast; no user intervention required.