Hacked MP3 player started by a motion sensor.
Helped out a friend doing an art class project. They are making an installation with paper tubes hanging from a roof. When you walk into the tubes a loop should start playing. Solved this with an old motion sensor from an alarm system, connected to an Arduino, connected to the PLAY button of a hacked MP3 player. The play button is connected trough an opto coupler just in case. There are some timing concerns to control the MP3-player, like giving the MP3 player time to play the loop all the way trough without new play pulses, therefore the Arduino.
The whole hack took me about three hours and was made mostly out of trash lying around my workshop.
Good luck, Yngvild