Re: Telephone Circuitry: Detecting Hangup

From: Clarence (no_at_No.com)
Date: 09/27/04 

Date: Mon, 27 Sep 2004 18:46:23 GMT

"Tomi Holger Engdahl" <then@solarflare.cs.hut.fi> wrote in message
news:lajfz54vz3w.fsf@solarflare.cs.hut.fi...
> "Zach Zaborny" <random@nts-technologies.org> writes:
>
> > Hello, for a project I am building a room monitor: you call the line and
the
> > relay picks it up upon getting the 90vac, and stays on. My question; how
> > would I go about getting it to detect a hangup?
>
> You must mean the hanging up on the caller end...
>
> The problem in this is that there is no single universal
> signaling to tell that the caller has hung up.
> The telephone system has been originally designed for humans
> that are intelligent enough to hangl up when call seems to end.
> On some telephone systems very clear signaling is available, some
> other not...
>
> > Is there a voltage fluxuation when the party hangs up?
> Usually not...
> > A tone?
> On some systems there are tones coming after some
> time after the caller has hanged up...
> > A polarity reverse?
> Some systems use polarity reversal to signal this I think...
> This is not widely supported feature.
> > It has been
> > troubling me for a while.

The signal your hoping for is intercepted by the Central office switch.
Nothing comes done the destination line until it times out as a dead
connection, (One end only connected) and that is the tone and voice warning
signal. To hear it just pick up a phone and hold it for long enough.