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From: jpexg@mit-hermes.UUCP
Newsgroups: sci.electronics
Subject: Re: I need a telephone ring detector chip
Message-ID: <2836@mit-hermes.AI.MIT.EDU>
Date: Fri, 17-Apr-87 21:20:18 EST
Article-I.D.: mit-herm.2836
Posted: Fri Apr 17 21:20:18 1987
Date-Received: Sat, 18-Apr-87 19:08:21 EST
References: <228@phoenix.PRINCETON.EDU>
Distribution: world
Organization: MIT AI Lab, Cambridge, MA
Lines: 15
Summary: Use an optoisolator

In article <228@phoenix.PRINCETON.EDU>, mzabeti@phoenix.PRINCETON.EDU 
(Mahboud Zabetian) writes:

> Is there a chip out there that would detect a telephone ring by outputing a
> signal?  Like have an output pin that goes high for the duration of each ring
> and then goes back to low.  

A quick and safe way to do this is to use an optoisolator (MCT-2 or IL-5 are
cheap and easy to find) across the line in reverse-parallel with a diode and
with the combination in series with a capacitor (try .01 mufd or so, and use 
bigger ones till it works). This is a good setup because telephone ring
voltage is ~90 volts--not what your logic wants to see, and you also don't
want anything you might do to zap the phone line. You'd need a 1-shot on 
the output (or a suitable filter) to make the transient blips from the 
isolator into a steady signal. Make sure the capacitor can handle >90 volts!