DIY Optical Theremin
This is a pretty simple projet for anyone who has a little bit of electronics experience. I used a circuit designed by Michael Una, Check out his website where you can look at his schematic and buy premade Beep Its here: http://una-love.com/munablog/category/beep-it/
I built mine for about $20 in parts from Radio Shack. Although the parts could probably be attained cheaper through the internet, I'm impatient and don't like to wait for shipping. It took only a few hours to assemble. I laid it all out on a breadboard to ensure it worked right, then started moving the pieces into a permanant enclosure. Instead of a petri dish, like Michael used, I mounted my beep it inside an empty 25 CD container, that blank discs usally come in, it opens easily for modification or battery changes, and stays locked pretty well too.