Many of the real "cheapo" novelty items (Christmas music and Halloween noise
makers) use a photocell to detect light changes.  An RC high-pass filter
keeps slow light changes from triggering it.  It's VERY easy to fool
however... (lights, wind blowing objects, small animals, etc.)

At 11:06 AM 3/4/96 -0500, you wrote:
>Harrison,
>
>What are your cost goals for sensing someone?  I need to do a similar thing
>but mine is battery powered and a novelty gift (i.e. power consumption and
>cost are most important).  How does ultrasonic fit in power and cost?
>
>Mark A. Corio
>Rochester MicroSystems, Inc.
>200 Buell Road, Suite 9
>Rochester, NY  14624
>Tel:  (716) 328-5850
>Fax:  (716) 328-1144
>e-mail:  Mcorio@aol.com
>
>***** Designing Electronics For Research & Industry *****
>