I'm working on a project to better understand how my home cooling system behaves. I'd like to produce graphs of temperature in various areas, overlaid with when my existing thermostat turns the A/C compressor and fan on.
Connecting up a few ESP8266's with temperature sensors reporting over WiFi is straightforward enough; I'm comfortable doing that. But I'm struggling with how to detect when my thermostat is calling for A/C cooling.
My (battery-powered) thermostat has red, white, yellow, and green wires:
- Red has ~28Vac
- White enables the heating system
- Green enables the fan
- Yellow enables the A/C compressor
With a multimeter, I easily determined that the thermostat simply closes a relay (I can hear it; that was a dead giveaway) and there's no resistance. So if A/C is called for, it closes relays connected Red to Yellow (and Green).
I don't really want to mess with drawing power from the 28Vac; it's easy enough to power an 8266 with 5V from a wall wart and that's sufficient for this project. Ugly, but acceptable.
I want to detect when the thermostat relay is closed and turning on the air conditioner. Clearly I'm not an EE by trade, and I'm struggling with the right search terms to communicate "detect whether two outside pins are electrically connected". I think something that emulates a multimeter's continuity function would work.
I did find references to the LM*39 series, but (1) I'm not sure it would like being exposed to AC power, and (2) the 'offset voltage' is usually measured in mV, which suggests to me that I'm barking up the wrong tree.