I just added "Free Play" to my Spy Hunter cabinet and I believe this method could be used for any arcade game. I built a small capacitive switch using the QT113 chip. It basically works like a touch lamp does. I register a coin credit by touching a piece of metal on the cabinet. I wired the sensor to one of the four bolts that holds the coin basket in my machine. The circuit I built is extremely small. I mounted it to the inside of the coin door and it gets power from one of the 12V lamps. I even tested the circuit as a proximity sensor through a 1/4" of glass - so if you don't have any metal to mount the sensor wire, you could just place it behind the arcade glass and wave your hand in front of it to register a credit. I posted a video of it here!
http://www.youtube.com/watch?v=I70iaR66QtE
http://www.youtube.com/watch?v=I70iaR66QtE

