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I apologise in advance if this is the wrong forum, or I'm not providing enough details.

I'm building a thermostat zone controller on a raspberry pi and I've encountered an issue with the relays in my Nest thermostats - mainly that they are 240v AC relays, which I would like to detect the state of using the GPIO on the raspberry pi (which are DC voltage at either 3.3 or 5v). So - I want to read whether the relay is open or closed (calling for heat or not). The details of the relays are listed as: "They are designed to work with wires with an AC voltage measuring 100-240V at 50-60Hz with a max current rating of 3A or with low voltage (dry contact) systems."

The Nest "Homelink"'s relays are a bit of a black box in that I don't know what specific component they actually are, but if I assume that they are a SSR's, then from what I understand the relay will turn on with dc current across it, but will not be able to turn itself off.

Given this, is it possible to trigger the relay's normal operation by some other method, such as cycling the GPIO on and off, so that there is no current across the relay, or even simulating an AC current via setting analog voltage levels (and swapping the gnd and positive sides at the appropriate time).

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  • \$\begingroup\$ Are you talking about relay-coil inputs or relay contact outputs? \$\endgroup\$ – ThreePhaseEel May 12 '16 at 3:53
  • \$\begingroup\$ I'm talking about the outputs. I want to effectively read whether the Nest thermostat is calling for heat, which it does by closing the relay. \$\endgroup\$ – Mal Curtis May 12 '16 at 4:05
  • \$\begingroup\$ Do you want the relay to drive other loads as well, or just the R.Pi's GPIO? \$\endgroup\$ – ThreePhaseEel May 12 '16 at 11:43
  • \$\begingroup\$ Just the gpio. The rpi then runs some other relays for the fireplace and zone dampers. \$\endgroup\$ – Mal Curtis May 12 '16 at 21:42
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In the Nest Installation Guide, page 23, they show an installation example with low voltage. So I think they actually are mechanical relays, or at least relays that can actually work with DC.

So you can make it simpler: connect 1 or 3 to the raspi ground, and 2 to some raspi input with a pull-up, for example.

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  • \$\begingroup\$ Cool, that's exactly what I've programmed - I just didn't want to plug things in and find that I'd completely misunderstood the documentation (they don't mention AC or DC so I wasn't sure). Thank you. \$\endgroup\$ – Mal Curtis May 12 '16 at 21:41

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