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From: Paul Burridge
Subject: Re: passive circuit excitation
X-Newsreader: Forte Agent 1.8/32.548
Date: Sun, 05 Jan 2003 01:04:35 +0000
NNTP-Posting-Date: Sun, 05 Jan 2003 02:32:24 GMT
Organization: Virgin Net Usenet Service
On 4 Jan 2003 15:33:58 -0800, email@example.com (Sara Berg)
>Joe, thanks! but in this scenario, how can i differentiate between the
>sound of the phone and other loud sounds? and what if i want to do it
>without a mic - isnt there anything that can respond to a specific
>frequency (not a sound wave necessarily), perhaps a transponder?
You just need a sensitive enough inductive pick-up (clue: "coil") plus
a couple of amplifying stages and an independent power source (9v.
battery or whatever). You don't need a tuned network to respond to a
specific frequency since you can position your pick-up close enough to
the ringing current field to be bandwidth insensitive. Neither do you
need extraneous sound to activate it. It's all done with those magical
E-M waves. :-)
"What is now proved was once only imagin'd"
- William Blake, 1793
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