Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!wuarchive!zaphod.mps.ohio-state.edu!mips!prls!pyramid!unify!csusac!mmsac!jim
From: jim@mmsac.UUCP (Jim Lips Earl)
Newsgroups: sci.electronics
Subject: Re: How Do I Make a Phone In-Use Light?
Message-ID: <5349@mmsac.UUCP>
Date: 26 Jun 90 05:10:19 GMT
References: <2204@sauron.Columbia.NCR.COM>
Distribution: usa
Lines: 24
In-reply-to: wte@sauron.Columbia.NCR.COM's message of 25 Jun 90 18:10:56 GMT

>What I *am* looking for is a simple circuit to let me know if any phone on
>my home line is off hook, preferably by lighting an LED or any sort of 
>lamp.

I haven't figured out how to do it, but I know it can be done.  A
friend of mine bought a desk phone that has just what you are asking
for - a "in use" LED - that lights up when any phone in the house is
picked up.  And here is the best part: it works off of phone line
power!  No batteries, or AC adaptor.  What I'd need to make such a
thing work is a device that works "opposite" of a zener diode.  That
is, it only conducts when the voltage across it gets BELOW a certain
level, rather than ABOVE a certain level, like a zener does.  So when
the phone voltage dropped below, say 10 volts or so, it would conduct,
allowing the line voltage to flow through it and a LED+dropping
resistor.  The only reason I can think of that this wouldn't work, is
that if it drew too much from the phone line, it would keep the line
"off hook".  
 
If you ever figure it out, let me know!
-- 
      Jim "Lips" Earl             UUCP: ucbvax!ucdavis!csusac!mmsac!jim
          KB6KCP              INTERNET: mmsac!jim@csusac.csus.edu
   =======================================================================
                  The opinions stated herein are all mine.