I live in an apartment with central heat. It's controlled by a round Honeywell thermostat. The heater ticks really loudly and annoyingly when it heats up and cools down (probably a topic for a separate thread), and the Honeywell thermostat causes it to heat up and cool down too frequently for my liking.
My plan is to replace the Honeywell thermostat with an Arduino board that'll have better hysteresis, plus show a more accurate temperature readout. (The Honeywell thermostat's current temperature reading is way off.)
There are two wires going to the Honeywell thermostat: a red and a beige. From my research (http://diy.stackexchange.com/questions/8019/is-there-a-standard-wiring-scheme-for-ac-heater-thermostats), that should be the standard hot and common at 24V.
However—and this is why I am writing—when I measure the voltage between those wires with a voltmeter, I end up with 0 V. The heater works fine with the Honeywell thermostat, so I cannot explain this. I have not tried closing the circuit manually, since now I don't know what to think.
Any ideas?
Best Answer
That will be a fun little project. I wouldn't get overly concerned about the voltage on those wires. You're safest bet is to use a small relay to isolate that part of the circuit. Choose a relay with contacts rated for 120V and 1A and you should be good to go. This will allow you to be very flexible in your choice of coil voltage and how you drive the load with the Arduino (there are plenty of examples about how to do that).
If you really do want to measure voltage, make sure the thermostat is not requesting heat (set it all the way cold), and set your meter for AC first... if you do get a reading, you can test again with DC to see whether you have an AC or DC source.