Path: utzoo!utgpu!water!watmath!clyde!att!pacbell!ames!netsys!vector!nobody
From: kaufman@polya.stanford.edu (Marc T. Kaufman)
Newsgroups: comp.dcom.telecom
Subject: Re: sensitive relay wanted
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Date: 28 Sep 88 15:07:41 GMT
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Reply-To: kaufman@polya.stanford.edu (Marc T. Kaufman)
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In article  wheels@mks.UUCP (Gerry Wheeler) writes:

>I'm looking for a relay to place in series with a telephone which will
>open (or close) when the set is offhook.  The closest I've come is with
>a 200 ohm 4.5 mA relay.  It didn't affect the phone operation too much,
>but wouldn't quite pull in.  If I pressed the armature in, it would
>latch.  Close, but no cigar.

You want to use a voltage comparator IC to measure the voltage between the
wires (through a full wave bridge rectifier, so you don't have to care which
way you hook it up).  Open circuit (on-hook) voltage is about 48, off-hook
voltage is about 1-2 volts.  I am not sure what "hold" voltage is, but it is
higher than 2 volts.  Simpler, for a single phone, is to see if the phone has
a spare set of contacts on the hook switch (many do), and use them.

Marc Kaufman (kaufman@polya.stanford.edu)