My brother and I spent a long weekend together in a hackathon-like fashion to create a theremin-like instrument using an ultrasonic sensor. The user waves his/her hand in front of the sensor to chose a note, and can adjust the octave using a knob. There is also a record mode which creates a loop that the user can edit on-the-fly.

Some photos of the final product:

The source code and circuit diagram can be found here:

https://github.com/mddub/theremax/

This project was made with the help of my brother, Mark Wilson. Check out his site: warkmilson.com

A video demo of theremax:

A time lapse video of the creation of the theremax: