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I am not an electrical engineer, just an enthusiastic DIY home owner. I would like my A/C to self turn on at 8pm, that is before I get home and have the ability to cut off power automatically. I have Sonoff basic devices doing this with my lawn lights, pool, etc. I have a 1 Channel Inching /Self-Locking WiFi Wireless Switch 5V 12V that's been sitting here unused. I connected it to one zone of my sprinkler system and it worked as expected—when the sprinkler controller turned on that zone I could turn on/off the Sonoff WiFi switch which then turned on/off that zone for so long, then off.

I thought it's summer, turn the HVAC selector to AC, find the power line, cut it, insert them in this low voltage Sonoff, and I can turn on/off AC on my phone or just use a set timer. The thermostat is a Honeywell RTH110B. Here is installation manual. Sonoff device is powered by cellphone charger or 12vc dc connectors.

Is it possible to integrate this Sonoff to smarten this thermostat and have the ability to turn on A/C in summer, heating in Winter?

picture of the PCB

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    yes. you need one relay per control channel (AC/heat). you can get cheap non-sonoff dual-relay ESP8266 modules on fleabay. the relay just replaces a switch (or mercury bulb), so play with your old themostat's mode temp settings while you probe the wires with an ohmeter to determine what wire does what; it's not a complex digital signal or anything... – dandavis Jun 21 '18 at 5:45
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Yes, this is something which ought to be quite simple. It isn't quite an off-the-shelf packaged solution at wither the control or the physical side, but there is nothing too difficult.

You need to work out if it is best to tap in to the main AC line, in which case you need to consider the peak currents and how switching will interact with the AC. Tapping in to the circuit used by the thermostat is a good idea, it removes any concerns about high power switching (although you need to treat the circuit as mains live unless you know it is safely isolated (not just low voltage).

If you tap in to the thermostat, your AC will never be completely off, but you don't need to worry about it having a cool down cycle after it stops running at power.

To switch the mode of the AC based on ambient temp will require a little more software, and you might also chose to make the control more intelligent so if external is close to your target, it may not be worth running the AC. (i.e. bias it towards not running in sprint/autumn).

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