Re: Phone line interface: Replacing a 1:1 transformer with an optocoupler

OBones wrote:
Ross Herbert wrote:

Olivier,
Provided that you don't have any DC current flowing in a 600:600
transformer they don't need to be big and clumsy. Since you say you
only want to "detect dtmf" then you should be able to arrange your
transformer with a blocking capacitor in series with the primary
across the line.

Well, a regular phone line has a DC component, if I'm correct.

It does, but you don't have to pass DC thru the transformer to get audio off
the line. In fact, this is a good way to monitor a line without placing
much load on it.
http://www.tkk.fi/Misc/Electronics/circuits/tele1.gif

Of course this all relies on another phone (or something) to allow DC to
pass on the line to bring the phone line to off hook status. There's allot
of decent information about different ways to interface to the line right
here:
http://www.tkk.fi/Misc/Electronics/circuits/teleinterface.html

A 220nF 250Vdc film cap and a Tamura MG-21 (600:600) miniature audio
transformer which has a 3dB freq range from 100Hz - 100kHz would be
suitable. The transformer measures only 8mmx10mmx11mm.
http://www.tamura-ss.co.jp/en/electronics/trance03/pdf/mg_mx.pdf

I trust you on this, but how do I access this for a one off series,
when I generally take the parts from the local hobbyist shop?
I'd rather use the optocoupler isolation as it uses readily available
parts that even come up in SMT packages, which are definitely smaller
and easier on the layout than a through hole part.

Let us know how that works out, I'm very interested. I've never seen it
done that way, but it may work fine. You're going to have to place a pretty
severe load on the line to do it though IMO. I prefer the DC blocked
transformer with clamping diodes myself. Watch out for those ringing
voltages.

I dug up a circuit for a dtmf monitor here
http://www.glolab.com/freeinfo/kdial.pdf and it uses a 100nF cap in
series with primary across the line.

Thanks for that.


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