The Theremin, if you haven’t heard of this type of instrument, is a device invented in the 1920s that is played without physical contact, and makes a sound like the voice on the original Star Trek theme song. If you’d like your own, you can now have one, via the Open Theremin kit on Tindie.
This kit takes the form of an Arduino UNO shield, as well as other supplied components. You will need to supply the actual Arduino board, as well as antennas since they, per the product’s description, they are easy to obtain, and bulky to ship. Once assembled, this digital Theremin senses the distance of each of your hands from the two antennas, creating its characteristic sound, as seen below.
Besides its nominal use as a high-pitched instrument, it should also be noted that you can change the sound characteristics around via a wave table on the Arduino. This should make it possible to map very different sounds, perhaps for a sort of “bass Theremin,” or something even more interesting. For that matter, since it measures hand distance from the antennas as a digital value, there is no reason you can’t use this for other non-musical applications, opening up the possibilities even further!
Note that there are two listings for this on the Gaudi Store, which appear to be the same, and are priced as such.