Here are instructions for creating a simple “break-beam” light detector circuit that virtually taps a button on the hacked device of your choice (music player, DVD) whenever an object interrupts a light beam (laser, flashlight). With slight modifications you can make it sound-sensitive via a line-level audio input.
Schematic, parts list, and PCB layout are included with Radio Shack part numbers inside the following files:.
NOTE: Over the years, I’ve come to the conclusion that analog electronics are too unpredictable and complex for beginners. This trigger circuit works, but a microcontroller would be a much better choice (fewer parts, more reliable, much easier to modify in the field). In recent years I’ve been directing first-time electronics experimenters straight over to the Arduino even if their application is simple. I think programming is a more worthy investment of time for folks who aren’t planning on an EE degree. Typing “delay(1000)” sure beats swapping resistors on a circuit board!