Goal:  To detect the number of people in the room.
Details:  Needs to send a 'count' trigger to a PIC16f628 (which
controls a bunch of solenoids) as people enter and leave a gallery
space (with possible distances up to 30ft).  Preferably, there is no
active device on the receiving end and no need to wire anything back
to the main device.  This is for a pedestal mounted art device that
will need to be moved and installed in different locations.

I've been looking at wiring two pen laser elements to beam across at
ankle level to two phototransistors (or photoresistors).  I would then
measure the beam break sequence to determine if the person is entering
or leaving.  Very prone to error, but absolute precision isn't all
that critical.  I am considering using a battery powered box that
would affix to one wall and beam back to the trigger box.  Reflecting
the beams back from a mirror seams too hard to calibrate, but I'm open
to ideas.

Any clever ideas out there?  Thanks in advance!
 - Damon

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