Wired to Wireless Thermostat

Looking for advice/how-to take an existing thermostat that is wired to AC unit and use Arduino to make that a wireless connection.

Simple Example: One Arduino would have 8 wires coming out of it and going to the AC unit and another Arduino would have 8 wires coming out of it and going into the thermostat.

What I want to do summary: Use the existing thermostat wires that are ran to power both Arduinos and thermostat(1 on the AC unit and the other on the Thermostat). All other wires(2 Stage Heating & Cooling, Dehumid, etc) that can be used want to use Arduino to send those wireless between AC unit and Thermostat.

Problem I am trying to solve: Existing Thermostat wire that is ran is only a 5 wire run. I am needing an 8 wire run including Common/Power. Instead of running new wire, want to use existing wires to send power to Arduinos and thermostat so I don't have to use batteries. Then use Arduinos to send wireless communication for the other wires I need.

I am hoping I explained what I am trying to do in a nutshell above. Have a little programming experience and know how to wire the AC unit.

Any help would be greatly appreciated!!

Without knowing the interface to the thermostat, it is very difficult to make suggestions.


Does the interface matter if it uses the standard wires(G, C, Y, Y1, W, W1, Dehum, RC) to communicate with the AC/HVAC unit? For example, you can swap out for any thermostat and the AC unit will still work.

These "standard wires" mean nothing to me. We need to know what voltage, data etc is associated with each wire.

If you have such information, please supply it.


get a wireless temperature sensor. there are tons that use common frequency for indoor/outdoor applications. using an arduino for one temperature sensor and then to send that out for remote monitoring is not a great use of resources.

make sure your logic has a wide dead-band, say 2 degrees. you cannot control a house to close temperatures and you will chew up your unit if you turn it on and off all the time.

most people do not have the ability to discern a 3 or 4 degree temperature change, but EVERYONE knows if they are hot or cold. and the same person sitting or moving will perceive the temperature to be too hot or too cold.

WeedPharma - Thank you for your assistance. Didn't realize that was what you were wanting to know. The voltage that can travel through any of the wires is 24 volts but that could possibly fluctuate +- 3 volts. Essentially the Thermostat will close the circuit depending on what is needed at the time. I am just needing to send that information to the Arduino that is connected to my HVAC unit itself. I have listed a great PDF below that does a good job on explaining how the circuits close. http://www.epatest.com/store/resources/images/misc/how-a-thermostat-operates.pdf

dave-in-nj - I am actually looking for a commercial thermostat to handle what a thermostat does but I am looking for arduino to handle the communication (i.e., closing the circuits depending on what the commercial thermostat sees fit) between the HVAC unit and Thermostat.

I have attached a Paint drawing I did. Don’t laugh. :confused: To help visualize what I am looking to do. The Arduinos would communicate with one another wireless. The common/ground is essentially the 24 volt power.

Thank you for everyone that looks at this post as well as thank you for anyone who provides feedback/direction.

As the lines are 24v, you will need to isolate them from the Arduino in both directions.

Inputs would be best via optoisolators though voltage dividers can be used. The outputs would need isolation via transistors (MOSFETS?).

Are you wanting two way comms? What Tx//Rx are you using?

The data sent would be best as a package of values at once, IE send value on each wire in one package. This is sent whenever there is a change. You may want a watchdog signal to confirm comms are operating correctly.