TR21 reverse engineered - Strooom/SmartTherm GitHub Wiki

With some luck, I was able to find the schematics of the TR21 on the web TR21 schematics It's pretty simple to understand how this works:

  • it's built around an LM324 quad opamp
  • two opamps are used to generate two voltages (~9 and ~15V) which are fed to a potentiometer which sets the wanted temperature
  • a thermistor generates a voltage for the actual temperature
  • the two voltages are compared and the difference is amplified to yield the 0-24V signal, for a temperature gap of around 3 degrees.