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

OBones · 2007-04-11T20:17:35+00:00

Hi all,I'm designing a system that uses a MT8870 DTMF decoder to detect tones on a phone line and the typical phone line interface is done via a 1:1 600ohm "telephony" transformer, like this : -. ,--------o------ To MT8870 Line )|( | )|( z -' '-. A | | Gnd | V z | GndThe two zeners are here to ensure the voltage does not go above Vcc applied to MT8870.My problem is that the transformers for these applications are bulky and take a lot of space. Hence I was wondering if I could replace the transformer by a diode bridge and optocoupler pair like this: Vcc | ___ .----------. | +-+----|___|---|-. .-|-' A A | | |/ | Line -+ | | V -> | | -(-+ | - |> | A A | | | | +-+------------|-' '-|-- To MT8870 '----------'My main concern is the frequency that the optocoupler can support. I have a bunch of 4N25 here and considering the max rise time and fall time with a security margin, I'd say it can transfer signals up to at least 30kHz which to me is enough for this application as DTMF signals only go up to 2kHz.Can anyone confirm that my calculations are right, and that the arrangement of the transistor side of the 4N25 is fine?Thanks a lot for your help.CheersOlivier

Apr 11, 2007 22 Replies

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Jim Thompson 19 years ago

I've often pondered, but never tried... has anyone tried to pick up the signal from the CURRENT in the loop?

...Jim Thompson

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Mark Zenier 19 years ago

Michael, you sound like you need a vacation.

Anyway, if the OP is looking for an circuit example, the October 1981 QST, reprinted in the 1991 ARRL Handbook for Radio Amateurs has a optocoupler based phone interface using several 4N28's and a couple of 741's.

Mark Zenier snipped-for-privacy@eskimo.com Googleproofaddress(account:mzenier provider:eskimo domain:com)

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OBones 19 years ago

Excellent! But how do I get a copy of this? I'm not living in the US right now, and I'm not sure where to start looking on the web. Could anyone share a copy with me?

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