The traditional theremin used a couple oscillators, a reference one, and another one was controlled via physical proximity. The difference frequency between the oscillators was then sent to an amplifier, whose volume was controlled by yet another set of oscillators. In the 1920′s such an instrument was no doubt difficult to build. In our time and place, while it would be easy to build with any number of IC’s, proximity to antenna sensing is a lot more challenging due to the huge number of RF sources in our environment. Case in point, last Friday night, our guitar player left her Ipod on my amp… it decided to call home, and man was it ever loud!
Many folks have built optical theremins. They are easy to build with low cost components. In a nutshell, a 555 timer could have its frequency controlled via a light sensitive resistor. The output of the 555 would then be amplified with a simple op amp circuit with another light sensitive resistor as part of the feedback loop. One big problem with optical theremins is their sensitivity to back ground light. Ie if the lighting changes, one also has to change their hand positions. This sensitivity to ambient lighting makes consistent performance challenging, and also can make it a difficult instrument to master.
Long Wave Infrared Radiation
My proposed solution uses long wave infrared sensing of human body parts. Specifically human skin radiation sensing in the 8-12 micron range. As skin radiation in this range is fairly consistent once a human has become thermally acclimated to their surroundings, such should provide a more consistent operation. In addition, my solution has automatic compensation for background radiation, albeit the actual process of compensation will be slow, and there are some higher order effects which can come back to haunt the musician. Apart from those, it is hoped that my design will provide a shorter path to mastery of the instrument in comparison with previous designs.
The 555 timer is key
Years ago, I designed a control system using the LM555… cheapness was one of the most critical design criteria, and low cost processors were still many years off into the future. In the process of developing this control system, I learned a number of unique things about the LM555 operating as a pulse position modulator… such is what I hope to leverage in my theremin design.
Timing is going to be tricky… I just came across the deadline for the 555 contest this morning. Entries need to be in by March 1. As such, this weekend should be busy, as I’ll not only have to design the unit, but also build it, test it, and even try to learn to play it. (and somehow or another figure out how to make a video and put it up on YouTube)