Status update since last log entry

After getting power handling resolved in last log entry, I started to really think how amazing the existing Honeywell thermostat was. It's been years since the last time I changed AA batteries in that thing and it's still chugging merrily along. I "knew" that it was only a 2-wire hookup, but I started to doubt myself - wondering if perhaps it's actually pulling power from the furnace. As a simple diagnostic, I shut off the furnace switch - and the Honeywell is still displaying. I also double checked and sure enough, only two wires are connected. It is still possible that the Honeywell is drawing phantom power normally (the way some smart thermostats can) and storing it in a supercap - or it could just be a very efficient design that can run an LCD years from a pair of AA batteries. While I was checking this, I noticed that I can actually send 24V AC from the furnace, and frankly that makes the thermostat requirements much simpler (no need to run when the furnace has no power, so no need for multiday standalone backup power). 

Redesign

Now that I don't need to run for multiple days without power, I am thinking that the space and complexity of two units within the thermostat housing is a poor tradeoff, so I'm switching from a Pi Zero W serial linked to a Feather to an ESP32 module (NodeMCU from HiLetgo specifically). I also picked up a cheap buck regulator for power. Feeding it from the 24V AC line through a bridge rectifier. The whole circuit is a lot more compact now. 

I am finding the ESP32 IDF environment a lot more complex than programming the Feather in the Arduino IDE.