Newsgroups: sci.electronics.misc
Subject: Re: Telephone Circuitry: Detecting Hangup
References: <[email protected]>
"Zach Zaborny"  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 votlage 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.

-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/