This is a quick, non-musical demo of the Therething, an open source MIDI theremin thing:

Ultrasonic sensors are used to return a distance value, from the sensor to the player’s hand. This value is converted by an Arduino or other microcontroller into a MIDI key value and is output at the correct baud rate (31,250 bps) onto a serial pin. Since MIDI needs a signal to turn off a key pitch as well as a signal to turn it on, two MIDI signals are sent – one to turn off the previous note and one to activate the required note. The new note-on message is sent BEFORE the previous note is turned off. This is to allow for a slide/portamento effect, making it sound more theremin-like on certain synths.

Volume is controlled using a similar approach, using an ultrasonic transducer that is pointing away from the note-generating sensor (to avoid cross-talk or interference).

See the BuildBrighton site for construction details.

via tubfun:

A quick demo of the new firmware for the midi theremin designed by the BuildBrighton hackerspace. See http://www.buildbrighton.com/wiki/Theremin for details.
Uses ultrasonic sensors hooked up to an arduino microcontroller. The arduino outputs MIDI which is then fed in to Ableton Live with an Operator synth set up.

via Make