Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!usc!samsung!think!mintaka!bloom-beacon!eru!luth!sunic!tut!santra!saha.hut.fi!c37189h
From: c37189h@saha.hut.fi (Harri Olavi Suomalainen)
Newsgroups: sci.electronics
Subject: Re: Wanted: Phone-in-use Indicator
Message-ID: <1989Dec18.101522.20175@santra.uucp>
Date: 18 Dec 89 10:15:22 GMT
References: <6375@nigel.udel.EDU>
Sender: news@santra.uucp (Cnews - USENET news system)
Reply-To: c37189h@saha.hut.fi (Harri Olavi Suomalainen)
Organization: Helsinki University of Technology, Finland
Lines: 20

In sci.electronics neumann@udel.edu (Brian R. Neumann) writes:
>
>Does anyone know of any simple circuit I could build or of any device
>that I could by to show when the phone is in use?

Voltage on the line is ~60 volts when line is free and ~10 volts while
the line is on use. You could bild up a simple device with only a few
transistors, fets etc. Notice that while the phone is ringing there is
~120VAC on the line! Be sure your circuit can take that!

Notice that you'll need an external power supply (eg. a battery) to
power your led. In case you'd take that "much" current from the line
your central office would find your line to be in use as if you'd
picked up a phone! 

Chears, Harri Suomalainen
--
      If you're feeling good, don't worry: you'll get over it !

         c37189h@saha.hut.fi           haba@otax.tky.hut.fi