This installment of "Weird Instruments" features touchless music, high-tech and sanitary.
Part 1: The Theremin
A theremin consists of two radio antennas, and you interact with it by moving your hands in the EM fields. One hand controls the volume; the other, the pitch.
It's what gives 1950's sci-fi movies their eerie soundtracks. For the most iconic example, refer to the classic soundtrack of 1951's "The Day The Earth Stood Still" by Bernard Herrmann.
In this TED talk, Pamela Kurstin demonstrates the theremin's versatility, making it sound like a musical saw and a bass.
Part 2: The Light Harp
Light harps (or "laser harps") come in all sorts of styles, but they're all played basically the same way: by interrupting or reflecting beams of light with your hands. The light beams substitute for strings and either trigger a synthesizer directly or produce MIDI codes to send to an external synth.
Here's "Theremin Hero" performing the Tetris theme on a reflective Laser Harp.
There are several basic approaches to building a light harp.
1. Scan with a single laser using a rotating mirror (similar to the way a scanner works at the grocery store checkout). The note is indicated by where the beam is when your hand reflects it to a sensor.
2. Use multiple laser sources and one (or few) detectors. It's similar to method one, but suitable for compact models. Basically, power is swept across multiple LEDs, and whichever one is turned on when the light is reflected is your note.
3. Use multiple detectors. It can be done with one scanning laser, or many individual ones. Since laser LEDs are so cheap, It's not terribly costly to use a bunch of them. I like this one approach because you don't necessarily have to scan, and I think you're likely to get better response, and it could manage chords. I also like relying on hardware more than software.
Whatever approach is used, the light harp can either directly control a synthesizer or act as a MIDI controller.
For the technically inclined, here are instructions on how to make a sophisticated laser harp (by Stephen Hobley). Steven points out that technically it's not a harp, but a lyre. I point out that technically it's neither, as it has no strings.
For the not-so-technically inclined, here are instructions on how to make one with an Arduino processor and simple breadboarded parts from Radio Shack.