Subject: From: Date:

RE: [ECS] Telephone Deadman switch gbailey Wed, 26 Aug 1998 17:37:07 -0600

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A phone line is supplied with -48V DC through 900 ohm series impedances on
both sides of the line. A normal phone on-hook is about 1300 ohms (the
ringer) and about 600 ohms off-hook. You can figure out the voltages under
various scenarios from there, BUT...
A dead line has 0V on BOTH sides of the line.
That is fairly easy to detect :)
Remember though that a ringing phone is seeing 90V RMS (around 300V peak to
peak) at 20 Hz. Enough to zap unprotected circuitry, fingers, voltage
regulators and other thingies.

Garnet Bailey                           gbailey@telusplanet.net
www.telusplanet.net/public/lmad    www.emgplace.com


-----Original Message-----
From:	Dan Carrington [mailto:dc_grafx@microworks.net]
Sent:	Wednesday, August 26, 1998 9:42 AM
To:	ECS newslist
Subject:	[ECS] Telephone Deadman switch

I am interested in building a deadman switch for my phone lines to watch
them for my security system.  I know how to make one for the
electricity, just hook a wall wart (transformer) to a relay and watch
the relay with a digital or analog input.  If the power shuts off or is
cut off, I can see the relay change state.  (this is assuming my
automation is on a UPS, which it is)  But I would like to have a way to
see if the phone lines have been cut.  I realize the phone system has
power supplied on it, I believe 70 volts AC.  Does anybody know of a way
to tap into this to see if the wires have been cut without messing up
the phone system?  My end want?  I would like an alarm system that does
not annoy the neighbors at all cost.  If my alarm system detects
problems it pages me and I can take appropriate action.  It flashes
lights, talks to the intruder, and sounds an in house siren (x10).  I
would like to install a battery opperated outdoor siren that will only
sound if the power and phone are cut and the alarm system is tripped.

Dan Carrington

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