Mylar is a good insulator and you're probably seeing a capacitive coupling when they hit. Off the top of my head, my sugestion kind of mirrors NCcams, an op amp in the first stage, LM321 or LM324 something that has extremely low input current requirements. Power it with the same supply as your basic stamp and experiment using a gain of 100 to 1000 and see if it would interrupt your basic stamp. Usually a micro has an edge sensitive interrupt. If you can get it to interrupt it reliably, then make your interrupt routine do what you want. i.e. drive a relay or....
Phil, Still too many interests, too many projects, and not enough time!!!!!!!!
Vist my websites - http://pminmo.com & http://millpcbs.com