Realization of coupling DTMF to telephone line

A project log for Pulse to Tone Converter

Converts pulse dialing of your POTS telephone to DTMF dialing allowing you to use it with your VOIP's FXS-port.

Christoph TackChristoph Tack 01/18/2018 at 20:412 Comments

Schematic explanation

Warning: This schematic is not a definitive version.  Only the DTMF-generator has been implemented.  There are no provisions yet for detection of pulses during dialing.

The schematic above couples DTMF to the telephone line.  


The measurements show the voltage at the audio output of the SLIC.  As you can see, most of the HF-noise has been removed and there's no noticeable deformation of the DTMF-waveform.

DTMF-pulse on SLIC's audio output
Zooming in on DTMF-signal on the SLIC's audio output
Picture of the current setup

There's not so much to see really.  The breadboard contains the Adafruit Protrinket 3V and the SLIC.  The 1.27mm-protoboard on the left has the opamp circuit on it.  The 2.54mm-protoboard on the bottom right contains the TLV431 circuits.  On the right, you can see the RJ11-connection to the telephone.


Christoph Tack wrote 01/20/2018 at 14:19 point

Like you said, there's no challenge in that.  There's commercial hardware that does exactly what I'm trying to do here in this project.

An alternative approach would be to use single chip tone generators and audio coupling transformers.  Easy to do.  Any kid can wire some ICs together.

The fun lies in creating solutions with a small library of "standard" components that are used over and over again in different projects.  Most of the times, this involves some analog circuit design.  That's the beauty of electronics hardware.

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Ted Yapo wrote 01/19/2018 at 20:14 point

I like the discrete approach!

You probably already know that you can buy commercial DAA's that will all the line interfacing for you.  At least you could at one point.  But, there's little challenge in that :-)

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