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I would like to create a solution (Probably with Arduino) to control my AC remotelly.

Opening my thermostat and doing some tests I got these values bellow:

Blue (C) - Neutral (AC)
Red (R) - Phase 28v (AC)

Just Fan
    - Green (G) 28v

AC On Running
    - Green (G) 28v
    - Yellow (Y) 28v
    - Orange (O/B) 28v

AC On Not Running
    - Orange (O/B) 28v
    - Yellow (Y) 0v

Heat On Running
    - Green (G) 28v
    - Yellow (Y) 28v
    - Orange (O/B) 0v

Heat On Not Running
    - All 0v

Orange (O/B)
    - If high (28v) is cold mode
    - If low (0v) is heat mode

Yellow (Y)
    - If high with Green (G), AC is running (cold or heat)
    - If low with Green (G) high, just fan is running
    - If low with Green (G) low, AC is off

My thermostat: My Thermostat

Is it possible to control my AC just joining these wires

For example:

  • Join green and red wires and just fan turns on.

Thank you and sorry any english mistake.

  • So... You want make a WiFi thermostat? I think Honeywell beat you to the punch, by about 20 years. – Tester101 Aug 30 '15 at 12:52
  • I know this. But I don't have $100 to spend now but I have Arduinos, Raspberrys, etc. I want to do this just for fun. – Victor Santos Aug 30 '15 at 17:20
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Yes, that's all any thermostat does. It connects the red wire to one or more of the others to turn on fan, heat, cooling.

The old mechanical thermostats did this with a simple glass bulb full of mercury fastened to a coiled spring. The temperature would make the spring length change slightly which would change the angle of the bulb, which would make the mercury run down to the end with the contacts...connecting Red to green (fan) and yellow (in your case, heat pump).

And yes, for your example, most simple thermostats have a way to make the fan run manually...that's all they do. Hook red to green with a simple switch. Fan stays on.

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