It worked. Then, it followed a flashlight. Then, it followed a car's headlight. Then, it didn't work anymore!

Just kidding about the car headlight part. :) I constructed that at a summer camp for geeks. I didn't do any of the design... the instructor handled that. But, we got to etch the boards, solder the components, debug it, and have fun. At that camp we also made holograms, and built an elevator controller with a microcontroller. I rate geek camps for kids very highly!