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From: fish@ihlpa.ATT.COM (Bob Fishell)
Newsgroups: sci.electronics
Subject: Re: I need a telephone ring detector chip
Message-ID: <3630@ihlpa.ATT.COM>
Date: Thu, 16-Apr-87 17:03:20 EST
Article-I.D.: ihlpa.3630
Posted: Thu Apr 16 17:03:20 1987
Date-Received: Sun, 19-Apr-87 09:28:23 EST
References: <228@phoenix.PRINCETON.EDU>
Organization: AT&T Bell Laboratories - Naperville, Illinois
Lines: 24
Summary: Use an opto isolator

> 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.

The best thing to use for this application is an  opto-isolator, so that
your circuit has no direct electrical connection to the phone line.
Any non-FCC approved device connected to your phone line is illegal, by
the way, so the following is offered in theory only.

The ringing voltage is about 90vac, and is applied at 20Hz, 2 seconds
on, and 4 seconds off.  You will want a DC-blocking diode and a
suitably large resistor in series with the isolator's input, so as to
bring the current down to within the specifications for the device. If
you want  a pulse for each ring, connect a large capacitor accross the
output of the opto  isolator through a suitably large resistor. Choose a
combination  with a time  constant greater than ~50ms and less than ~1 s.  
You can drive CMOS with this  directly  (OK, you've got lousy rise
and fall times -- CMOS is forgiving). If you want to use TTL (though I
can't imagine why), you'll have to  shmitt-trigger the output to clean
it up.
-- 
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/  \				Bob Fishell
\__/				ihnp4!ihlpa!fish