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From: dt@yenta.alb.nm.us (David B. Thomas)
Newsgroups: sci.electronics
Subject: Re: How Do I Make a Phone In-Use Light?
Message-ID: <1558@yenta.alb.nm.us>
Date: 26 Jun 90 16:53:24 GMT
References: <2204@sauron.Columbia.NCR.COM> <5349@mmsac.UUCP>
Reply-To: yenta!dt@bbx.basis.com
Distribution: usa
Organization: yenta unix pc, Rio Rancho, NM
Lines: 32

jim@mmsac.UUCP (Jim Lips Earl) writes:

>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 secret is not voltage but current.  An led is placed in the loop so
that it turns on when loop current flows.  When the phone is off hook,
that subcircuit isn't connected, so the led is off.  This works for one
phone only, however.

With voltage comparators, it is easy to make a circuit that simply monitors
the voltage on the line, lighting and led if the voltags falls between,
say, 5 volts and 20 volts (typical off-hook voltage is 9 volts).  I have
built and used one of these, and will happily send schematics.  This method
of course requires external power (I used one of those little wall power
supplies), but what is neat about it is that you can use the two unused wires
in the phone cable to carry the led signal, and just install an led and
resistor at each telephone.

Another neat phone circuit (VERY simple) uses an scr, a couple of resistors
and a button.  If you wish to switch phones, you hold in the button (gating
the scr), then hang up.  The scr maintains loop current.  An led could be
included in series to show that the line is being held.  Then you can take
your time, walk to any phone, and pick it up.  The extra load on the line
will lower the current through the scr to the point where it unlatches
itself.  No more extra trips to switch phones.  You need one of these at
every phone you are likely to answer, but not want to talk on.  I'll send
this circuit to anyone interested as well.

					David (yenta!dt@bbx.basis.com)