mfontana@eniac.seas.upenn.edu (Mark A Fontana) (06/20/91)
Hello all,
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.
Thanks,
Mark Fontana
mfontana@eniac.seas.upenn.edujon_sree@world.std.com (Jon Sreekanth) (06/22/91)
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