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From: robf@mcs213f.cs.umr.edu (Rob Fugina)
Newsgroups: sci.electronics
Subject: Telephone "line in use" indicator"
Keywords: schematic, telephone
Message-ID: <2231@umriscc.isc.umr.edu>
Date: 22 Feb 91 22:47:40 GMT
References: <154@datatec.DATATEC.COM>
Sender: news@umriscc.isc.umr.edu
Organization: University of Missouri - Rolla
Lines: 32

I took ideas from schematics posted here a few days ago and constructed
a telephone "line in use" indicator.  Here's the circuit...


                       1M           1k        /   E  (to +5)
   >-----        -----\/\/\----+---\/\/\-----|
                               |               \   C
                               \               |
                               / 220k          \
from                           \               / 1k
phone      bridge              |               \
line                           |               |
                               |              LED
                               |               |
    >------      --------------+---------------+
                                               |
                                              GND

The transistor is a PNP Motorola 3638 with hFE of around 100 (probably
doesn't matter).  Also, you could use this with different supply voltages
if you change the 220k resistor.

Also, in case anybody's interested, I found the on-hook open-circuit voltage
of my phone line to be 48.7V, and the short circuit current to be 72.8mA.
This leads to the conclusion that the line has a resistance of about 670
ohms.

If anybody has any ideas of how to do this without an external source,
please get back to me...


Robf  robf@cs.umr.edu


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