Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sdd.hp.com!think.com!snorkelwacker.mit.edu!world!jon_sree From: jon_sree@world.std.com (Jon Sreekanth) Newsgroups: sci.electronics Subject: Re: Need "line in" circuit for telephone Message-ID: Date: 22 Jun 91 06:11:02 GMT References: <44873@netnews.upenn.edu> Sender: jon_sree@world.std.com (Jon Sreekanth) Distribution: na Organization: The World Lines: 31 In-Reply-To: mfontana@eniac.seas.upenn.edu's message of 19 Jun 91 23: 35:27 GMT In article <44873@netnews.upenn.edu> mfontana@eniac.seas.upenn.edu (Mark A Fontana) writes: I'm working on a project that requires a direct audio connection to the telephone line (to speak to the user, play music from tape, etc). The level of the signal will be the standard "line out" level found on most home audio equipment. Could someone describe a circuit to handle this connection? I would like to ensure that the audio source is suitably protected, of course, and a volume control is essential. You could use a 1:1 transformer to couple to the telephone line. Make sure that the primary can take 110 mA of DC without saturating, and has 300 ohms or less of DC resistance. Connect the primary across the telephone line, connect the secondary to your audio output source. You might require some amplification. Adjusting the level is best done by experimentation. For protection, put a 360V MOV (Metal oxide varistor) across the primary, and a much lower voltage (say 10V or so) MOV across the secondary. About 1nF across the secondary is helpful. This way, your audio circuits don't see more than +- 10V or whatever you pick. There all kinds of telephone toy schematics regularly published in Radio Electronics, and other such magazines. There's lots of background material there. / Jon Sreekanth Assabet Valley Microsystems Fax and PC products 346 Lincoln St #722, Marlboro, MA 01752 508-562-0722 jon_sree@world.std.com