Although the theremin was invented nearly 100 years ago, the instrument still has enthusiastic fans. It is played entirely without physical contact, and almost everyone will have heard one at some point. The original design from 1920, by Lev Termen, alias Léon Theremin, used vacuum tubes, but JFETs can equally well be employed. In this article we look at the development of an experimental design for a theremin that you can build yourself.