When the building looses power, and then eventually turns back on, our portable A/C unit doesn't turn back on. You have to manually push the button on the front of the unit, or hit the power button on the remote. Our A/C unit happens to be in our server room, and bad things happen when it's turned off for too long.

I put together a few simple devices to turn the A/C back on in the event of a power loss. It persistently tries to turn the A/C unit on, and won't stop until it sees that the A/C unit is turned back on.

Parts you'll need:
Christmas light day/night timer - $15
MK111 interval timer - $15
12v dc power adapter - $?
12v dc buzzer - $3
Tape -$?
spare wire - $?

You'll also need some basic soldering skills to put the MK111 interval timer kit together and solder a few extra wires in the remote control, and the Christmas light timer.

Step 1: Hacking the Lighting Timer

I had an old Christmas lights day/night timer laying around. This was the first step for me in creating this. I don't have the detailed step / pictures that went into this, but it's fairly strait forward.

This is your typical "Christmas Light" timer. It has a "Dusk to Dawn" feature. When it senses there is no light, it will power the plug at the bottom, and when it sees light, it turns the plug off.

First I ripped out the light sensor (photocell) and soldered an extension for it so I could attach it to the A/C unit. The other reason to rip is out was that  I needed to isolate it from any outside light.

Then I taped the photocell to the "cool" LED on the A/C unit. This LED light is only on when the unit is on. I used black electrical tape to make sure it could only see light from that LED. Then I used the stickiest tape that I could find, and taped over that. I also taped the wire down for a short distance in case somebody snags on the wire.

Once that is complete, this circuit will only turn on if the A/C unit is turned off. And once the A/C unit is turned on, the plug will turn off.

Next is the device that turns the A/C back on.

Easiest way to increase time between cycles would be to swap out resistor R1, which should be 1k, with a 10Meg resistor. That should give you about 10min cycle time. If leakage of C3 is too high that time could be considerably less.

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