It's great how Derryn uses an off-the-shelf PID controller and replaces a single board with his custom design.
Because the design of these controllers is modular, with a power supply board, display board and separate microcontroller board, I've designed a new microcontroller board that has the same physical form factor of the old board. I'm using a PSOC 4200 micro because it has some nice analog stuff for this applications, and I've got a bunch sitting around from when they were on a promo sale for $1ea. The board is designed to slip straight in with a small amount of soldering and make it somewhat of a universal controller.
Source: Modding a cheap PID controller via Hackaday
Learn more at: http://paxinstruments.com/hack-cheap-pid-temperature-controllers/