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I want to control my home thermostat using an Arduino microcontroller. From what I've heard, it is pretty straightforward as the Arduino can be powered through the 24V that is coming to the thermostat and you can switch the A/C, heat, and fan through a relay. I don't want to mess directly with the thermostat without a little guidance from maybe someone who has done it before.

I have solid state relays on-hand, Arduino boards, and also any other components that are required can be purchased. Any pieces of advice or guidance for this project? How do I make the connections between the relays and the thermostat wires?

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    \$\begingroup\$ Slightly off-topic: Apart from the hardware, be aware that your software needs to do some anticipation. This is caused by the fact that once the room temperature is reached, the radiotors will still be hot and therefore heat the room further. In old mechanic thermostats you actually had an anticipation resistor under the bi-metal, to slightly heat the bi-metal a bit above room temperature while the contact was open. \$\endgroup\$ – jippie Mar 13 '13 at 19:36
  • \$\begingroup\$ I got this already as the thermostat I am trying to design is a self learning thermostat, similar to that of the Nest thermostat. Thanks for the advice though. I am really looking for advice in building a controller unit (relay box with Arduino that controls HVAC outputs). \$\endgroup\$ – tabchas Mar 14 '13 at 2:27
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It looks like the "accepted" way to connect the thermostat wires is to use a bi-stable, or "latching" relay. The thermostat in my house uses the Omron G65SK-2-H-3 relay. It looks like its a 3V, low signal, latching relay that can handle the 24v connection from the thermostat wires (which, if I understand correctly, controls a bigger relay that actually supplies power to the unit).

The reason for using this special kind of relay seems to be that thermostats are usually battery powered. So if you use a regular relay, it will drain that battery as it holds the relay open. If you use a latching relay, you just send a quick pulse to the relay and then it holds in that position until it receives another pulse.

If you look for these relays on ebay and digikey, they seem to go for about $3-$5 each. After some searching, I found a pack for 10 for about $2 each. The searches I used were: * "5v latching relay" * "low signal latching relay" " "G6SK-2-H" (although I did not end up getting this exact relay)

If you design the thermostat so that it plugs into the wall, you probably don't have to worry about using this special kind of relay.

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