Wall Air Conditioning Control

I am wanting to control my bedroom AC with an arduino that has a temperature monitor attached. The AC unit only has a knob to turn it between AC/heat/fan. I plan on leaving it in the AC position and using a relay to turn the unit on or off.

Would something like this work for me? The AC is plugged into a 20A outlet, so I need at least that current rating.

I plan on having it turn on or off only every half hour or hour, so the compressor should be fine.

before you can select a device to use, you have to know the rating of the motor. I have to assume that in a small unit, the motor in the compressor is not over 1 hp. but you must calculate for that.

second, if you are going to add any controls, add two, one for the fan and one for the compressor.
add a temperature sensor on the output of evaporator. on the copper tubing. if the tubing is cold, there is cold that can be used to cool the space. once the compressor is stopped, the high side of the compressor will equalize pressure, while the pressure decays, cooling is available. keep the fan on while cooling is available.

set the room temperature setting to control the compressor start and stop. this is where you add the time delay to prevent short cycling (starting the compressor into a high pressure )

it is not effective to turn the whole unit on and off.

Ah. I was planing on just having this be an extra step between the AC plug and the wall outlet; cutting power conpletely when I turn it off. It sems like that is not recommended?

We live in an apartment, and the AC unit is theirs, so getting into it and modifying it isn't something I am comfortable with doing. Would it controlling it still be possible without getting in and modifying its wiring?

Thanks for the help!

you still need to get the total power of the unit.


if it is 220VAC, you might be able to use something like this. you would need to get a short appliance extention cord and cut it to get both a plug and a receptcle end.

It does seem odd that the unit has no temperature setting.

What about a semi-mechanical approach? Can you remove the plastic knob to reveal a spindle, and find a way to attach a servo to the spindle? Either directly, or using gears or levers, for example something like this.

Sounds a bit "Heath Robinson", I realise, but perhaps you could hide the servo (along with the Arduino) inside a box which fits entirely over the original control box and is screwed to the wall. The box could have an lcd display and an IR remote control receiver mounted to show through the cover.

This would avoid the problems & dangers of switching the mains supply, because you make use of the unit's own switches/relays.

When you move out of the apartment, you can simply remove the outer box and re-fit the knob, assuming you haven't lost it by then!