This Instructable shows the steps I took to build my own open source theremin (Open Theremin) when there was none available to buy online. The lovely thing about open source is that the plans are freely available to make and modify.
A theremin is an electronic musical instrument that you play waving your hands in proximity of its two antennas to modify the sound.
The Open Theremin is an Arduino based open sourced hardware implementation of the instrument designed by Urs Gaudenz in Switzerland. The open theremin PCB is a shield installed on top of an Arduino.
What led me to make this was I was looking to get an Open Theremin for my nieces birthday gift but at the time they were all out of stock. Here is a video of the build and short demo of playing it for first time.