Hacking the MP3 player

I took apart the MP3 player and did some measurements on the switches.

It's pretty convenient that I can detect power-on through the switch voltages. Whew. For playing I'll need to check that there's some signal on the output which could be a bit tricker. I think I'm going to compare to some arbitrarily low voltage and run the comparator output through a diode to a capacitor. The capacitor would discharge slowly with a resistor through the base-emitter junction of a transistor: the collector could be attached to a resistor to 5 volts — while playing, it would stay very close to 0 volts (logic 0) but if there was no ouput, it would slowly rise to 5 volts, eventually tripping whatever logic input.

I wired up little wires to the switches and battery case to hook it to the logic circuits and power. What a pain to solder to this surface-mount stuff: