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.

Step 2: Making the Interval Timer

I was trying to find some sort of timer/relay that would connect 2 wires together at an interval of a minute or so, and I came across an instructable for time lapse photography. They used this same timer kit, and I realized that this was exactly what I was looking for.

I bought it from www.cs-sales.net. The model nubmer is MK111. $6 for the kit, and $7 shipping.

This kit has an adjustable 555 timer in it that can click 2 wires together anywhere from 2 - 60 seconds. It's powered by 12v dc. It comes disassembled, and you'll have to solder it yourself. I took a 12v dc power adapter from an old set of HP speakers to power the device.

Next we need to wire the interval timer to the remote control.

Step 3: Wiring the Remote Control

I took the remote control apart and found the two leads from the power switch, and soldered a pair of wires to them.

Then take the other end of those wires and insert them into the relay portion of the timer.

Step 4: Putting It All Together

I was lucky and had a phone panel to screw everything to. It helps keep it nice and clean.

Now when the power goes out, the Christmas light timer will see that the LED on the A/C unit is turned off, and will activate the interval timer circuit.

The interval timer circuit continuously hits the power button on the remote control.

Once the power comes back on, the remote will turn the A/C unit on.

When the photocell sees that the LED light on the A/C unit is on, it will turn the interval timer circuit off.

Wohoo!

A couple days after I finished putting it all together I realized that I can put a 12v buzzer in the system. I picked up a 12v 15ma 75db buzzer from Radio Shack. Now when the Interval timer circuit turns on, it makes an annoying buzz to alert me that it is running.

Step 5: Possible Problems and Future Plans

My Christmas light timer currently has to be plugged in at all times or it will loose it's settings. I have it plugged into a reliable battery backup, so I'm not too worried about it. There are other light timers that have a solid dial and don't lose their settings. When I find one, I will replace it with the current one. That way It can plug into the same outlet that the A/C unit is plugged into.

The batteries in the remote could die. Just make sure you have a fresh pair every now an then. I would have wired the relay switch directly to the power button on the A/C unit, but I would have to void the warranty on the A/C unit. I figured it would be easier to unsolder the wires off of the remote, should I ever need to get the A/C unit replaced under warranty.

And finally, if the photocell were to fall off or get pulled off, and it was dark in the server room, it would turn the A/C unit on and off every 60 seconds. Probably not a good thing for the unit, so make sure it is secured well! I installed the buzzer because of this reason. I would like to get the interval timer to go off every 10 minutes instead of every minute.

If anybody knows what component of the Interval timer I can adjust / hack to get the time longer, please let me know in the comments! I assume you 'up' the resistance in the potentiometer, but I'm not 100% on that.