Residential thermostats in my area boil down to two effective subsystems:

  • A set of relays to control which features of the HVAC system are on.
  • Programs to monitor and activate those relays based on some predefined set of rules.

Perhaps I missed something however the price of thermostats, especially smart thermostats, seems absolutely silly. Really my desire is for a relatively unintelligent node controlling the relays and move the smarts into something like Home Assistant. Probably a bad idea, but I bet I can get below the $60 for an internet connected device.

Hilariously while researching parts I came across a Espressif Board containing a temperature and humidity sensor.

Component Usage Price
ESP DevKitC (v4) CPU/MCU $7.595 ($15.19 for 2)
4 channel Songle SRD-05VDC-SL-C HVAC signaling $7.99


  • NodeMCU style design. Back plate for incoming wires with drill holes. Might be too small.
  • Wall Mount Case can be scaled up to fit all electronics. Need to modify box to fit hole.