> 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.

I think noise would be your enemy with this, A big lump of steel a few
inches above a sensor is much easier to detect with inductive sensors
than a few pounds of water several feet away. You'd need to carry some
resonant device.

Other possibilites?


You could have an IR transponder consisting of an IR receiver, some
small intelligence - or even something hardwired - to give a return
code, and an IR diode. Each room has a similar IR transceiver that polls
the room. If you're in there your transponder signals back. It could
easily be made into a badge, pen-type device etc, minimum of processing
required, individual IDs available. Unambiguous indication. Best of all,
pretty cheap to implement.



Hmmm, or how about hacking a serial output from a casio GPS watch
 http://www.casio.com/gps/ ) and sending that to your system via IR, RF,
induction loop or whatever?
Neat if it could be done. If e.g. the receiver is separate from the
display driver and not all under one big epoxy blob.
Not cheap, possibly not accurate enough in all cases, but cool as
anything...



> Seems to be even more practical now if you start setting up all this
IR
> detectors all over the place.

Then there's I-Buttons.

Or you could have a coded electromagnetic badge or hang-tag like some
building security systems use.

Or barcoded ID Cards, or wristbands. There are surplus supermarket POS
barcode readers
for sale cheaply online, I'm sure I've seen some recently.

You'd only need one pickup per room for a simple toggle system, and a
bi-colour led to confirm if it thinks you're in or out.

With a wrist or belt worn tag you could have an in sensor and an out
sensor on either
side of the doorway if you wanted positive log-in/log out switching.

Hmmm, this is getting silly. I'd go for the IR transponder, cheap,
cheerful, small, simple to install and relatively unambiguous.













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