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From: coltoff@burdvax.UUCP
Newsgroups: sci.electronics
Subject: Re: How can I detect Telephone Line in Use.
Message-ID: <3430@burdvax.PRC.Unisys.COM>
Date: Mon, 11-May-87 10:47:57 EDT
Article-I.D.: burdvax.3430
Posted: Mon May 11 10:47:57 1987
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Keywords: telephone

With all the other phobe questions I was going to post this today anyway.
This is from the TI LInear and Interface Circuits Applications book, 1985,
pgae 7-23. The circuit is as follows. Use a TLC271 Op amp (available
at Radio Shack). There is a 5.1M resistor from each phone wire to pins
2 and 3 of the op amp. Across the phone lines is a 4.3M resistor and
a 0.01uf cap. Pin 3 of the 271 is connected to ground through a 2M resistor.
Pin 4 goes right to ground. Pins 7 and 8 are tied together and then to Vcc.
(In this case it is 9v). Between pins 2 and 6 there is a 2M resistor and
a 0.001uf Cap in parallel. Pin 6, the output, goes through a 1.5M resistor.

HOW DOES IT WORK DR. SCIENCE?

Well Rodney, your ignorance of telephony appauls me. When the phone is off
the hook a 1K load is placed between tip and ring. This removes the
differential input to the op amp and causes its output to go low. When
the phone is hung up the impedance is removed and the differential voltage
across the line causes the op amp output to go high.

The TI example uses a TCM1520 as a ring dectector. This chip is designed
for just that purpose and needs 2 resistors and 2 caps to interface to the
phone line. Radio Shack sells a phone ringer, the TCM1512, I believe
which can also be used as a ring detector.