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      Pressure-sensing mats are widely available commercially.  See any issue
of Sensors magazine for ads.  Such mats are widely used to control automatic
doors, and are very reliable and not outrageously expensive.  Fail-safe mats
are available for machinery-guarding applications.  This is the standard
approach.  Tapeswitch is one vendor.

      If you want to detect motion, rather than presence, as in an alarm
system, you might look into Kynar film, a DuPont product which generates
electricity via piezoelectricity when squeezed.  If you're thinking of
a security device that is hidden under carpet, this could be an effective
approach.  It has the significant advantage that static weight will not
cause output, and therefore heavy carpets, even with furniture on them,
won't cause false triggering.