For the Ottawa Mini-Maker Faire this year, [Steven Dufresne] created an electronic take on the classic mechanical music box.
A typical music box uses a sequence of bumps on a rotating drum to pluck the tuned teeth (called lamellae) of a metal comb. Steven ditched the drum and comb and replaced them with a strip of paper and a single 555 timer. The timer is configured as an astable oscillator with a fixed capacitor and charging resistor. The discharge resistor is selected via a series of 13 shaped wires that drag along the strip of paper. When a wire drags over a hole, it is connected to a copper pad below that is soldered to a specific resistor. This completes the circuit and generates a tone specific to the resistor selected.
While the electrical aspects of the project are fairly simple (not even requiring a circuit board), the mechanical parts are much more sophisticated. Steven had problems getting the dragging wires to make good contact and keeping the paper roll pulled tight. He outlines all of these challenges and how he solved them in his very thorough video summary (embedded after the break). With all of his incremental improvements to the design, the finished music box stood up to a whole 14 hours of abuse at the Faire.