There's really nothing Nest-specific in the design. It just takes two components (AC input optocoupler and resistor) to sense the 24VAC output from any standard

There's really nothing Nest-specific in the design. It just takes two components (AC input optocoupler and resistor) to sense the 24VAC output from any standard thermostat. Connecting to the Nest was probably easier than turning an ESP32 into a proper thermostat. The hardest part of the whole project was figuring out how to read a clean signal from a photodiode.