Theremins are starting to show up everywhere.

Popular Science has a fun DIY project that promises to let you make a theremin that operates on changes in light:

Note: Unlike a conventional Theremin which operates on two radio frequency oscillators, this Pocket Theremin consists of an oscillator and frequency divider that are manipulated by changes in light. Be forewarned, the Pocket Theremin is very light sensitive and must be “played” in subdued lighting for achieving the best sound effects.

According to PopSci - the project should take about 3 hours to build, is easy and should cost less than $20.