How does a discotheque laser work and how could I build one? — JPD, Spiennes, Belgium
If you are referring to a system that displays illuminated line drawings on a wall that move with the music, then building one is easy. You need a small isolated speaker—just the electronic device, not a whole speaker unit—that you can connect to the music amplifier. Place an elastic membrane over that speaker—a stretched sheet of thin rubber from a latex glove should work well. Then glue a tiny, front-surface mirror to that rubber membrane, choosing a point that is about midway between the middle of the speaker and its edge. A front surface mirror is one that is shiny on its top, so that light doesn’t have to go through glass before reflecting. A broken fragment of mirror, about 3 mm on a side, should work. Finally, shine the beam of a laser pointer onto the mirror and begin to play music through the speaker. The mirror will move with the music and the reflected laser beam will form pretty patterns on the wall.