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From: plate@dicome.UUCP (Douglas B. Plate)
Newsgroups: sci.electronics
Subject: Re: How do you build phone off-hook indicator
Message-ID: <1562@dicome.UUCP>
Date: Thu, 10-Sep-87 09:19:46 EDT
Article-I.D.: dicome.1562
Posted: Thu Sep 10 09:19:46 1987
Date-Received: Sat, 12-Sep-87 15:29:10 EDT
References: <12660@bu-cs.BU.EDU> <1005@cooper.UUCP>
Reply-To: plate@dicome.UUCP (Douglas B. Plate)
Organization: DICOMED Corp., Minneapolis
Lines: 42

Here is a circuit I thought might work with no external
power supply or batteries.  I don't know the on/off hook voltages
of phone lines, but with a little bit of figuring, this
should work for any voltages.  I just thought this up, I 
HAVE NOT tested it!  Maybe someone can confirm/reject my
theory of operation.  Here it is:

VCC=phone line voltage

	o		o
	|		|
	\		\
	/ R1            / R2
	\		\
	/		/
	|   R3  	|
A o--------/\/\/--|<-----    <--LED
	|		|
	--|	       ---
	/ |            / \  ZENER DIODE
	\ |		|
	/<| VR1 	|
	\		|
	|		|
       ---	       ---
       ///	       ///	

THEORY: The phone line voltage  is applied at the top of R1 and R2.
The zener diode selected must have a reference voltage lower than
the lowest (offhook) phone line voltage.  Adjust Variable Resistor 1
so that the voltage at the cathode of the LED is the same as the voltage
at the anode (zener voltage) when the phone is on the hook.  When
the phone is taken off the hook, the voltage at point A will drop,
while the voltage at the zener cathode will remain the same.  This
should create a potential that will foward bias the LED, causing it to light.
R1 and VR1 should be of a high enough value not to load the phone line,
R2 limits current through the zener, and R3 limits current through the
LED.  Varying the value of R3 will determine the brightness of the LED.
I am ASSUMING that the phone line will supply enough current to light an LED.
If I get a chance I'll try this out myself.

	Doug Plate