At 11:02 PM 7/5/97 EDT, you wrote:
>On Thu, 3 Jul 1997 19:50:03 +0200 Eric van Es <vanes@ilink.nis.za>
>writes:
>>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!
>>
>
>
>        Thanks for the responses...  Guess I didn't make the requirements
>clear.  What we're trying to do is make a very small transmitter and a
>very small receiver.  The receiver needs to let us know if the
>transmitter is "out of range" (about 10 feet away).  Distance itself is
>pretty noncritical.  since this is a very short distance for RF, I was
>thinking I could just do a damped sine wave generator by stepping a PIC
>output from 0 to 5 volts or back.  This output would probably drive a
>series LC tuned to some RF frequency (as allowed by part 15 of the FCC
>rules for unlicensed devices).  The idea is that at the resonant
>frequency there would be a high circulating current in the LC, causing
>radiation from the L as the current decays due to losses.  We're kinda
>pulling an RF harmonic off the step waveform.
>        That's kinda what I have in mind for the transmitter.  For the
>receiver, another LC driving something.  Perhaps a current input type RF
>amplifier.  The L would again serve as the receive antenna.  The RF amp
>would drive some sort of detector which would drive the receiving PIC
>where some coding would identify the unique transmitter.  this is a
>little like keyless entry stuff.  Could also use "transponder"
>technology, but it seems that a separate transmitter and a separate
>receiver is more cost effective here since they would always be sold as
>pairs.  I think transponders are typically used where there a bunch of
>real cheap transponders and a few relatively expensive units for polling
>them (like theft prevention tag systems).
>        So... anyone have some ideas in this area?  Cost is very
>critical, but if we are only transmitting a few feet, it doesn't seem
>that we should have to go to triple conversion superhet receiver design!
>
>Harold
>
It sounds a lot like an electronic (invisible, RF) pet fence to me. You
might want to buy one at any pet supply shop and reverse engineer it. Such
a fence is a transmitter emitting a low frequency (~150KHz) and when the
dog gets within a few feet of a wire buried in the ground around the
perimeter of the property, the dog gets a shock from a receiver in his collar.

Please forgive me if I tell you something you already know, but many people
on the piclist are foreigners and may not be familiar with such devices.
:-) I have designed a proximity detector (capacitance sensing) for a
client, but it only has a range of a few inches (all the application needs).

BTW I also have an electronic security device that is a keyring receiver
that beeps if my briefcase gets more than a few feet away. You can buy a
similar device at a baby supply store to alert you if your child gets away
from you while you are looking away. They can't be expensive. I hope this
gives you some ideas.