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From: atn@cory.berkeley.edu (Alan Nishioka)
Newsgroups: comp.dcom.telecom
Subject: Why Does Device Cause Ringing to Stop?
Message-ID: <16771@accuvax.nwu.edu>
Date: 6 Feb 91 14:27:09 GMT
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I've been trying to build a box to make key phone's lights flash and
to implement a hold button. I am using a 1M/5M voltage divider across
the line and some comparators to detect voltage levels and thus ring
and off-hook.  This isn't the best method, but I want it to coexist
with other phones on the line in parallel and to be adjustable (ie.
sans zener diodes) for now.

(I forget what you call these phones but they have fifty wire cables,
five line buttons and a hold button, ex-standard AT&T office phones)

The problem enters when I add a 200ohm resistor and a 2n2222
transistor across the line to implement the hold button. (Common
emitter, with a 47K base resistor to a CMOS latch output)

When I call the line, a phone wired in parallel chirps shortly (a
Ferrari phone, if that matters :-) and the ringing voltage ceases.
The ringback is still heard on the calling line and the called line is
still on hook and can be answered, but there is no ringing voltage.
When I wire the base of the transistor low (off), the problem goes
away.

Is this some sort of protection that is tripping because there is a
wierd impedance across the line?  What could be causing it?

My CO is a 1AESS, I think.  It has been a long time since they
installed it and offered tours.  You hear a clunk when call waiting
beeps.


Alan Nishioka      KC6KHV      atn@cory.berkeley.edu      ...!ucbvax!cory!atn
974 Tulare Avenue, Albany CA 94707-2540     37'52N/122'15W    +1 415 526 1818