I have listend to Metalica when playing Billard. And at one stroke, the solo part overlapped with it. I wanted to recreate this. Used a piezo buzzer as input (https://www.arduino.cc/en/Tutorial/KnockSensor). Which threshold value is 42 ;) This triggers the servo and picks the string. Here is a vimeo of it: https://vimeo.com/161231937

Version 2 is planned, with thinner body design.