I was browsing through my local Radio Shack and saw a really neat item.This can be used with or without a "real" security system on board.

It is a security warning sticker thingy with a flashing LED in the middle. Here is the package with the model number on it.

It normally runs on two batteries, but that wasn't good enough for me - nooooo. I wanted something that I didn't have to remeber to keep fresh batteries in. Since I was ALREADY in Radio Shack, I picked up a regulator.

I wired the regulator and the some appropriately sized resistors (instructions on the back of the regulator package) to a cigarette lighter plug and it all fit in the back of the battery case (after I made a cutout for the wire, of course).

Next, I needed a way to put it on the window without ruining anything. It normally has an adhesive (permanent, I guess) that sticks to the interior of a window to make it visible from the front side.

I used a piece of thin plexiglass (kind of hard to see in this picture) and stuck the unit onto that. Now I can just slip the whole thing down the window slot whenever I want it to be used.

I measured it and it draws around 40 MICRO (yes, micro) amps (!) when in use, so it can be left on basically forever.