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I thought I was all ready to replace a thermostat with an Arduino. I used a multi-meter and bridged some connections to discover the following:
1. bridging 230V (live) with Cool turns on the Aircon outside unit
2. bridging 230V with F2 (brown) turns on internal fan on LOW

Thermostat incoming wires

Now I would like to insert an Arduino into this circuit, powered from the 230V (using AC to DC module) and not from a battery. The Arduino will control a relay module. Then I realized I do not have a common ground for the Arduino (actually NodeMCU).

The only way to have power run through this circuit is to bridge 230V with either Cool, F1, F2 or F3. I would like to have the Arduino powered on even if the Airco / Fan are not on, since the Arduino should control the relay.

I have two questions:
1. can I use one of the Fan lines as a "common" since it is lower voltage than the live wire?

Like this:

How I would like to connect the AC->DC module

I assume that in this case there may be some "trickle" of power running from Live to Fan through the AC-DC module. As long as it does not consume a lot of power and does not break the fan I would not care too much. Is this right or am I making a mistake here?

  1. Are there other options to make this work, without running further wires or using a battery?
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  • \$\begingroup\$ It doesn't sound like you ready to replace the thermostat at all. Be careful when playing around with 230V. It might kill you. You need the common or "Null" wire for the AC/DC module. What are you using as reference, when measuring the voltage on the 230V wire? \$\endgroup\$ – Peter Karlsen May 1 at 9:05
  • \$\begingroup\$ Reference are the Cool / Fan lines, difference is about 230V. \$\endgroup\$ – John May 1 at 9:35
  • \$\begingroup\$ Are you sure that there is 230V present on these lines? Almost all HVAC systems use a 24V transformer for the control circuit, and connecting the feed to the fan or cool demand lines will be operating a relay in the control inside the unit, that switches the line voltage to the motors. \$\endgroup\$ – Phil G May 1 at 13:12
  • \$\begingroup\$ Honestly I think these are "logical" signals and there is not actually much current through the lines. I am sure the difference per the multi-meter between the "hot" wire and the other wires is about 240V when everything is turned off, or less when Cool/Fan are on. \$\endgroup\$ – John May 2 at 9:12
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It might be instructive to read this FAQ entry from Nest, makers of a smart thermostat, about the C wire:

https://nest.com/support/article/When-Nest-needs-a-common-C-wire#wire-may-thermostat

As far as I can tell, what Nest does when "power stealing" -- that is, operating without a C wire -- is charge an internal battery from power that it draws between the 230v and heat/cool wires. This is somewhat similar to what you're proposing, but note: (1) Depending on your fan, it might run while you "steal" power through it, or stop and start, or heat up (because it doesn't normally pass current while not blowing), or something else unexpected. (2) When you do want to call for the fan to actually blow, you will have to short together the 230v and fan wires, at which point you can't draw more power through them: now you see why the Nest has a battery.

The Nest engineers probably spent a lot of time fine-tuning this and playing with different models of heating and AC units to try to get it right; and they still have an FAQ entry noting that, when using power stealing: "in some cases, you may experience odd heating or cooling behaviors ... [y]our system is making strange noises: chattering, stuttering, clicking or thumping - this can be caused by your system turning on and off rapidly ... the system fan is always running or won’t turn on [or] turns on and off repeatedly in a short period."

So I would suggest that this is a tricky endeavor best left to the pros. Have you considered that most thermostats run on double-A batteries?

(Also note: Working with 230v is dangerous. It can be lethal. I feel obliged to note that it's not really a great idea unless you're an expert and meticulously following proper practices. Do you know the measurement category of your multimeter? If it's not Cat III, it's not technically even rated to be safe to measure mains power at the socket.)

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  • \$\begingroup\$ I am starting to see why most thermostats use batteries. My multi-meter says Cat I 600V and Cat II 300V, but not Cat III <= will be getting a Cat III meter. \$\endgroup\$ – John May 1 at 9:39
  • \$\begingroup\$ I am thinking to pull common wire from near-by light switch and/or outlet and use that, although ugly \$\endgroup\$ – John May 1 at 12:21
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    \$\begingroup\$ If you have a nearby outlet, it's much easier and safer to just plug the Arduino's power adapter in to that. 'Borrowing' a neutral connection from another circuit could create hazards and is unlikely to be allowed under the electrical regulations wherever you are. \$\endgroup\$ – nekomatic May 1 at 12:55
  • \$\begingroup\$ I agree this would be the safest solution and I consider this for one thermostat. The other thermostats only have light switches nearby, no electrical outlets. So I would have to make or pay someone to make an outlet. I might actually do that. \$\endgroup\$ – John May 2 at 9:16
  • \$\begingroup\$ Since I have multiple Fan lines I may still try to steal power through one of them, since I only use one fan setting. \$\endgroup\$ – John May 2 at 9:27

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