Dinner, with a control loop
I built a functioning sous-vide-style immersion circulator from a PID controller, solid-state relay, thermocouple, immersion heater, and submersible pump. It is a small collection of fairly ordinary components with a very particular job: keep a water bath at a controlled cooking temperature.
Each part has one job
The thermocouple measures temperature. The controller uses that feedback to regulate the heater through the relay. The pump keeps the water moving. Putting those parts together makes the cooking system: heat, measurement, adjustment, and circulation working as a loop.