Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!unmvax!deimos.cis.ksu.edu!uxc!uxc.cso.uiuc.edu!m.cs.uiuc.edu!irwin
From: irwin@m.cs.uiuc.edu
Newsgroups: sci.electronics
Subject: Re: Sensing telephone off-hook
Message-ID: <21000013@m.cs.uiuc.edu>
Date: 13 Apr 89 15:05:00 GMT
References: <22901@agate.BERKELEY.EDU>
Lines: 37
Nf-ID: #R:agate.BERKELEY.EDU:22901:m.cs.uiuc.edu:21000013:000:1766
Nf-From: m.cs.uiuc.edu!irwin    Apr 13 10:05:00 1989


/* Written 11:44 pm  Apr 10, 1989 by 128a-3db@e260-3b.berkeley.edu in m.cs.uiuc.edu:sci.electronics */
/* ---------- "Sensing telephone off-hook" ---------- */
>I realize this was asked recently, but I just started reading this newsgroup
>and I think I missed some of the responses.

>We need a sensor circuit to determine whether a telephone line is off-hook.
>I have lots of parts handy and I suspect it is a relatively simple
>voltage-range detector circuit, but I'm not sure of the voltages.  Is it
>this simple?

>The reason we need this sensor is to prevent somebody from interrupting
>a download in progress.  It would be more fun and slightly cheaper to build
>rather than buy if the circuit is indeed simple.

>Thank you very much!

>	-Jonathan Dubman
>	128a-3db@web.berkeley.edu
/* End of text from m.cs.uiuc.edu:sci.electronics */

At the church that I attend, there is a "privacy circuit" on the phone.
There are several extensions in the church, but if the Pastor's phone
is off the hook, all of the extensions are disabled, to prevent those
who might tend to eaves drop from doing so.

I was helping the pastor one day, making some changes in locations
of extensions. We decided to look into how the circuit worked and found
that there was a current sensitive relay in series with his phone. If
his phone was off hook, the line current picked up the relay, which
opened the contacts and the extensions were disabled. I put a meter
in series and found that the line current was about 45 mills.

I would think you could use this technique. Get a sensitive relay
and put it in the circuit, wire the extensions through the relay contact,
when you have the phone off hook for computer work, the other extensions
will be dead and can not cause you problems.