Fifty years ago, the December 1970 issue of Popular Mechanics showed how to make this “dancing light display”. The idea was quite simple, and employed no electronics. It simply used a string of Christmas tree lights made up of flasher bulbs mounted on the rear. These were projected through a cardboard with cutouts of various geometric shapes, only a sheet of plastic on the front. Optionally, a speaker could be mounted inside the cabinet.

Even though the changing of the lights was random, according to the article, when music was playing, it would give the illusion of being in time with the music.