Heat Exchanger
Mash Tun
PID Controller
The Control Box uses an Omega CN132 autotune temperature controller linked to a solid state relay and a J type thermocouple.

  The CN132 controls the heating element in the heat exchanger.  This is the brain of the system and it ramps the temperature right up to where you want it.  Once at temperature, the CN132 holds it there with very little variance, waiting for you to ramp up the temp controller for your next step.

The temperature is measured as it comes out of the heat exchanger, so your wort never gets hotter than the temperature setpoint.  Some RIMS systems measure temperature in the mash tun.  This is all fine and good, but the wort could be boiling in the heat exchanger while the contents of the mash tun are still coming up to temp.