I have an old PID controller I used for a blade heat treating kiln I built but I haven’t been making knives in quite a while. I have, however, been learning to brew and an accurate temperature control system would be really helpful and since I basically have all the parts on hand, why not?
PID controllers are amazing little devices because they learn how a system responds and adjust themselves on the fly. For example, my furnace was set to 1700 degrees F and you could watch the controller kick on and start adjusting when it powered the heating elements on and off until it would hold within a couple degrees. It really was amazing considering the unit was less than $30.
Anyway, here’s a good explanation of how to wire them up. The applications are only limited by your imagination.