This is a demo of Forrest Mims, III’s Audible Light Meter. This photo-theremin design was originally published in Optoelectronic Projects Vol 1, published by Radio Shack in 1976.

The schematic is included within the video.

via EA78751:

a super minimal photo-theremin from back in the day… lets build it and see what it sounds like.

it only has 7 parts including the speaker and battery!