This is the second reply to your post today.

After thinking about it, the answer came.

When I was a kid we would clip a playing card or baseball card
to our bycicle so that as the wheel turned the spokes in the
wheel would hit the card,  This would make a noise as we rode
the bycicles so that we 'played like' our bikes were a
motorcycle.

Take a piece of stiff plastic. attatch it to the end of a stick.
Place a microphone on the end of the stick near the plastic.
Then as you run the stick up and/or down the pile, it will make
a noise as it passes over the edges.  The signal from the
microphone can then be analized by the hand held device (held
in the other hand?) attached to the stick/microphone.

Bill C.   bill@cornutt