Some suggstions I have if yoiu want to make a simple coin validator. First, make the slot exact to accept say the P5 coin. you cannot insert anything with a larger diameter and thickness. the coin should roll on a track for a distance say 2-3 inches but the rail has slots so that if a smaller diameter coin in drop, it falls off downward before reaching the trigger sw. If it is the right coin, a micro sw on the rail is push down by gravity(of the coin) and it should be "one shot" so that the coin continous down to a pocket. The sw once trigger starts a timer. If all songs have equal time length, the timer can be fixed hut that is not the case. So,. there must be a way to stop the timer once the song ends. This could be a ckt which senses when the music stops such as when the volume drops for a minimum period of time.