I have a question on how to safely connect and detect Thermostats outputs to Controller's GPIO inputs (RasPi and BeagleBone). I've been looking up a lot and most of the cases it is showing me how to connect GPIO outputs through relays then Thermostat inputs to make it a smart thermostats. I don't want that. What I'd like to have is:
Thermostat -> Controller -> Furnace.
So, I'm currently having a relay board between Controller and Furnace to control it. This should work. However, on the input side, I'm not sure how to connect thermostats output wires to the controller and detect which one is energized.
In other words, I'd like to know what ? in my high-level below needs to be so that I can connect them safely and know which thermostat call is called, knowing that thermostat output voltage will be 24VAC, and GPIO input can take 5VDC/3VDC, I believe. Please correct me if I'm wrong.
Thermostat -> ? -> Controller -> Relay board -> Furnace.
Thanks a lot guys.