dc42:
On the connector, the ground connection is evidently the middle pin. Try connecting a 330 ohm resistor (or thereabouts) between +5V and one of the outer pins. Connect the other outer pin to an Arduino input with pullup enabled. You're looking for it to read LOW when there is nothing in the gap and HIGH when there is. If you don't get that, swap the 2 outer pins.
It worked! 8)
Thanks for the info!