skr@bellcore.bellcore.com (10/15/88)
Mr. Moderator:
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In article <telecom-v08i0150m11@vector.UUCP>, jetzer@studsys.mu.edu (jetzer) writes:
> In article <telecom-v08i0148m08@vector.UUCP>, mgrant@cos.com (Michael Grant) writes:
> > A long while ago someone posted a question about how to build a simple
> > circuit which would light a LED (or a simple lamp) when an extension
> > was lifted elsewhere in the house. The requirements are:
>
> > - The LED lights when another extension is lifted, showing that it
> > is in use.
> > - The LED must be powered off the phone line.
> > - No extra wires must be run to the telephone.
> > - The circuit should be reasonable small so that it can fit inside a
> > desktop style phone.
>
When I was in college a friend & I built a circuit to do just
this --- as I recall, it used a Zener diode (9V I believe) which
shorted the LED at the higher voltage when the phone was
on-hook, but stopped conducting at off-hook voltage, causing
current to go through the LED. Just put the Zener in series
with the LED and a resistor, and insert that in series on
the line (not sure if tip or ring, or if it makes any
difference). If that doesn't make sense, I have the schematic at home;
if anyone really wants it I could attempt to post it (could be
a challenge).
I must admit that it was my friend that thought up the
circuit (Chuck, are you out there?).
Steve Reinhardt
Bell Communications Research
Red Bank, NJ