>Okay, okay.
>
>I'm back to my original idea of monitoring the signal difference from the
>transmitting wire on (or in) one wall to the receiver wire on the other.
>Something like they do for traffic lights.  Signal changes when a body
>occupies the room.
>
>Seems to be even more practical now if you start setting up all this IR
>detectors all over the place.
>
>John Mullan

This sounds interesting ... I would love to hear about your results if you
get it to work.  I'm not sure how one would do this though.  Inductive
sensors for traffic lights are simple because cars are large metallic
objects.  The electromagnetic properties of humans aren't nearly so
convenient.

If the subject comes close enough to the antenna you could sense the change
in capacitance like a theremin.  But to cover an average-sized living room
may be difficult.  Perhaps you could make a big grid (approximating a plate)
under the carpet?  Seems like a lot of work.

PIR sensors may do the trick with a little modification -- how about using a
diffuse lense instead of the typical Fresnel lense and have a PIC directly
sample a high-resolution A/D reading from the sensor.  Any reasonably rapid
increase in reading could be interpreted as a human entering the room and a
rapid decrease could be interpreted as a human leaving the room (under the
right circumstances ... the software would have to reject changes due to
lights being switched on and off, etc., but, if it is part of a home control
system then it should know this information anyway).

Ken