My partner, Beth, asked me if I could make her a Theremin. So I have. It’s called the Betheremin.
A Theremin is a musical instrument, which changes pitch and/or volume as you bring your hand close to it’s antenna(e). The way this works is by your hand influencing the capacitance of a resonator circuit, changing the frequency at which it oscillates. This difference in frequency creates a “beat” frequency against a reference oscillator, which can then be used to create an audible frequency or control a Voltage Controlled Amplifier.