LM35 temperature sensor, reads from freezing to above boiling point. That can handle mash and boil temperatures as well as fermenting and conditioning in a single device. It wont interface directly to a Pi's GPIO so theres a microcontroller doing the conversion on the breadboard by the looks of it.
Cant tell if its a PIC or an Atmel but both have analogue inputs that can read sensors and digital ports that will talk to Pi's GPIO, and all three have community contributed code libraries to do these things.
The hardware as shown would read the temperature of the outside of the kettle, but its easy to put it into a bit of copper pipe to make a liquid probe. Thats all you'd need, with the software turning on the burner and chiller mentioned on their page they can keep the brew at the exact temperatures needed regardless of room temperature - its a proper microcontroller brewery...
I brew my own beer too, and theres a lot to be said for electronic measurement. Cant say I'd thought of automating it to this level though, I'm happy with digital timing, measures and temperature readouts to help my eye.
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