In order to avoid playing with my live thermostats, and to give me access to the hardware for testing at my desk, I have assembled a test rig. The configuration consists of a board with the thermostat and an incandescent light bulb connected using a standard, high amperage wall plug.

The light needs to be incandescent to provide a high enough amperage load on the triac in the thermostat. The wiring is to the spec shown in the manual for a 4 wire connection, therefore interrupting the black(hot) wire heading to the light.

With this in place I was able to use normal jumper wires to connect the high voltage board to a breadboard and then from there to the existing low voltage board. This allows me to test the connections between the two sides and intercept them where possible. The thermostat operates properly in this configuration.

From the labeling on the high voltage board, it appears that only three of the wires are used, providing:

• ground
• power
• signal

A few initial test were done:

1. Voltage between power and ground was approximately 6.8V on my multi-meter. Closer to the ATmega it read 3.3V
2. Connecting power directly to signal causes the light to come on. It appears that the power is triggered properly this way.
3. Trying to power a 5V Trinket from the power line did get some power, but not enough to boot the processor

At this point I need to figure out the power situation. The power regulator on the Trinket is able to take 6.8V, but I'm not sure that is what it is getting. I'm using a very basic multi-meter at this time. Once power is figure out, I will load up a basic blink sketch and see what happens. I may also test a ESP8266 that only needs 3.3V power input and see if that is sufficient to boot the processor. As that is my target, it should be fine to develop from there.