**NOTE: This device will not actually change traffic lights for you: it's no more than a glorified timer circuit. It's obsolete technology, but basically makes a 555 circuit slightly more interesting.
This is actually something I did over a year ago; it was kind of my foray into electronics. As you can imagine, it's fairly simple, but I thought I'd post it anyway.
First off, a little introduction. You may have noticed that emergency vehicles (fire trucks, ambulances, etc.) seem to always get a green light at intersections. They do this by using a mobile infrared transmitter. Basically all it does is send an infrared signal to receivers mounted on the poles the lights are on. When the receivers get an infrared signal of a certain frequency (I've heard 12 and 14 hertz), they turn those lights green and make the adjacent ones red. I've seen these sold for hundreds of dollars, but as you'll see, it's a ridiculously simple device you can make yourself for less than around $30.
Fortunately (for emergency vehicles) this device is obsolete, as most (if not all) of these systems now use a rolling code or other secure system. So this whole device is basically pointless, but hey, I had fun thinking I was a criminal mastermind.
Now, onto the build. I followed this article for the most part. It's pretty well documented on that site, so it makes my job here easy. The only differences are I mounted an indicator LED on the side of the case that flashes with the IR array, and I mounted the array itself on the lid of the enclosure with a small screw. I just used one of the project boxes from Radio Shack and it worked pretty well.
Here are some pictures of the final project:
The trickiest part of getting everything to fit was the power transistor (on the right). I ended up screwing it down into the case, which isn't the best idea for high-current loads, but it was only driving an LED array so it wasn't a big deal.