I go through old Everyday with Electronics mags every now and then. I
think I saw a article in one of the 1996 issues that went on about
location-sensing with inductive loops. I'm sorry that I can't be more
specific about which issue it was, but it sounded much like what you
want.

good luck!

Harold Hallikainen wrote:
>
>         I've been asked to look at developing a simple proximity
> detector.  I kinda have in mind using a PIC to pulse a series resonant LC
> once per minute or so.  The LC would radiate a damped 49 MHz sine wave
> based on its self resonance.  The PIC would just drive this LC with a
> step that changes state once per minute or so.
>         I need to detect this damped sine wave up to 10 feet away.  I'm
> thinking of another LC (probably using circuit board traces for L)
> driving a single chip RF amp set up as a Schmitt trigger so it only
> detects stuff over a certain level.  This would drive a diode detector
> which would then probably drive an analog in on a PIC.  Software would
> check the timing of the damped RF pulses to identify the particular unit
> transmitting.
>         Anyone have experience in this area?  I am somewhat concerned
> about the whole thing being jammed by a cordless phone but don't want to
> build a superhet receiver to try to get more selectivity.  Perhaps a
> ceramic or SAW filter after the LC?  Even then, with shared spectrum I'm
> likely to find interference.
>         Comments?
>
> Thanks!
>
> Harold

--
eric van es
vanes@ilink.nis.za
cape town, south-africa
http://www.nis.za/~vanes/