Trying to get a good solution for detecting square inch sizes of aluminum foil hidden in heavy cardstock.. Can't really use light because getting to the opposite side, for a backlight, is almost impossible. I thought of maybe using a small RF emitter and sensor at angles, so if foil was present it would deflect the emitted into the sensor. Or maybe a very sensitive metal detector would be able to pick it up?
I know what a metal detector is.. and the likelyhood of detecting paper-thin pieces of flat aluminum foil is VERY LOW.
The theft-detectors used widely in retail stores detect a small assembly of foil and plastic smaller than your little finger. And they detect it from 2-3 feet away. If you had the foil in the shape of a "doughnut" it would be self-resonant at some frequency and easier to detect.
I believe those retail theft detectors work on a different principle of passive harmonic re-radiation. It's normally a simple diode that has an input tuned circuit and a output tuned circuit at a different frequency, normally a higher harmonic of the input frequency. The fixed equipment radiates a constant frequency RF signal and 'listens' for the presence of a harmonic carrier frequency at the same time. If it hears a signal it knows that a tag is passing through. Makes for a very small and inexpensive tag.
You could get two needles, connect one to an analog/digital input, and the other to 5v, then jab them into the card stock to see if you get an electrical connection.
One might try using an rf shielding setup where the foil would shield a small antenna when placed in a test spot. Some times the DollarTree store has small earbud FM radios for $1. I think the antenna is in the ear bud wire. Coil up the earbud wire and put it in an aluminum foil covered jar lid (assuming plastic lid). Put a piece of aluminum foil over the top and see if the rock n' roll station goes away.
and the likelyhood of detecting paper-thin pieces of flat aluminum foil is VERY LOW.
That is not true, foil is picked up very well by inductive techniques equally as well as solid metal. It sounds like you need an inductive proximity detector. While not cheap you can get them. It all depends on the range of detection you need. The longer the range the more expensive they get. http://uk.farnell.com/jsp/search/browse.jsp?N=500006+1004304&Ntk=gensearch_001&Ntt=inductive+proximity&Ntx=mode+matchallpartial However it is perfectly possible to make your own. Are you good at winding large coils with thin wire?