The principal behind this is more like changing the ground plane of the
transmitter therefore changing the signal that is picked up by the receiving
antenna (remember, there is a transmitting wire along one wall and receiving
wire along an opposing wall).

There is actually a device that does this already.  They pump a signal
through the one wire and compare it to what is received at the receiving
wire.  They provide another connection if you need to put a metallic/foil
sheet under your flooring to increase the ground plane effect.

I have seen their page on the internet and will try to find it.

I don't want to just buy this though, I want to build something like it.

John Mullan


-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Ken Webster
Sent: Saturday, February 19, 2000 10:06 PM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: SV: Re: Home Automation with PICs [OT] (was: Internet
Toaster [OT])


>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