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From: raoul@eplunix.UUCP (Nico Garcia)
Newsgroups: sci.electronics
Subject: Re: How Do I Make a Phone In-Use Light?
Message-ID: <906@eplunix.UUCP>
Date: 9 Jul 90 16:32:43 GMT
References: <2204@sauron.Columbia.NCR.COM> <5349@mmsac.UUCP> <4546@milton.u.washington.edu>
Distribution: usa
Organization: Eaton-Peabody Lab, Boston, MA
Lines: 22

In article <4546@milton.u.washington.edu>, wiml@milton.u.washington.edu (William Lewis) writes:
>    When the voltage gets high enough (ie, off-hook condition), 
> the Zener starts to conduct, dropping the voltage oacross the
> LED to less than 1.6 or howevermany volts. Maybe another resistor
> directly in series with the LED would help... 
>    This seems simple enough, or am I missing something? 

You sure are: the phone company supplies ~48 Volts at their end of the
phone wires to an on-hook/OPEN-CIRCUIT line. This is what you will measure
on your phone lines. When you are off-hook, current flows (which used
to latch the relays in older phone systems) and the DC voltage on your
phone drops to about 10 Volts due to line losses. With your circuit, the
LED or the Zener will *always* conduct current, and the phone company
will never hang up your phone.

So remember: high voltage = open circuit = ON-HOOK. Otherwise the phone
company would have to supply current to all the hung up phones in the
world, a wasteful technique.
-- 
			Nico Garcia
			Designs by Geniuses for use by Idiots
			eplunix!cirl!raoul@eddie.mit.edu