At 15:44 02/07/97 EDT, 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.
>[...]
>        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?

Why don't you measure the inductivity instead of the damping of an LC?
Could be done e.g. with an oscillator measuring its frequency, or a bridge
circuit.

-------------------------------------
Gerhard Fiedler <gerhard@pobox.com>
S‹o Paulo - Brazil