The theremin is a fairly simple instrument conceptually, although by no means easy to construct. It consists of two oscillators (or wave-makers) that produce waves at frequencies that are well beyond the reach of human hearing. The human hearing range is between about 20 Hertz (20 Hz) and 20,000 Hertz (20 kHz), although the top end of that tends to drop out rather quickly with age. Anyway, the two oscillators produce waves in the megaHertz (MHz = 1,000,000 Hz) region. The mixer takes the two waves and puts them together. The resulting sound is the "beat frequency", and is in the range of human hearing.
One of the oscillators can be varied by moving a hand over an antenna. This changes the frequency of the outgoing wave, and therefore changes the beat frequency, or the sound that you hear coming out of the theremin.
These are the theremin chaps. The yellow trace on the oscilloscope is the output from their theremin.
One of the oscillators and the mixer.
Theremin chaps diagnosing what turned out to be a wonky transformer on their circuit. (Well, actually, the one in the pink shirt is talking to a girl who was trying to sidle into the picture.)
The Very Low Frequency (VLF) receiver picks up low-frequency radio signals that occur naturally. These signals are related to electrical activity, such as lightning strikes, in the upper atmosphere/ionosphere. These can be heard as crackles and crunches. It consists of an antenna, a filter that passes only a specific set of frequencies and an amplifier. My students tried out their circuit on a prototyping board, and they've just completed the portable version of it. They'll be trying to take some measurements outside this week.
A 2 kHz signal being passed through the receiver circuit.
Closeup of the circuit.
VLF receiver chaps admiring their handiwork. Students building a radiation detector out of a coffee tin futilely attempting to hide from the camera.