DTMF Tone Decoder

Obsolete project for reference only
This project has an impossible requirement or uses parts that are no longer available. It is here for reference or inspiration only. Consider it “unsupported”.

This project displays telephone numbers decoded from tones. Specifically, telephone touchtones. Dual Tone Multifrequency (DTMF) tones. This has nothing to do with CallerID.

A microphone picks up the tones, a preamplifier boosts the signals, an SSI-202 DTMF chip decodes the tones, a Basic Stamp acts as an interface to an LCD display and also provides “RS-232” serial output.

DTMF Tone DecoderDon’t try to build this project. It’s too expensive to do it this way, and some parts are obsolete. Dave Schmidt is making a reasonably priced DTMF decoder with similar features. Check out his web page: www.dschmidt.com/dtmf.html and see what his tone decoders are like. They look really good. If you’d like to buy a tone decoder, please don’t write me.

If you’d like to build your own tone decoder “from scratch”, you might have trouble finding the SSI-202 chip. Radio Shack used to sell them. B.G. Micro, www.bgmicro.com used to sell them and might still have a datasheet, but no stock of the part. Silicon Systems, maker of the SSI-202 chip, has been dissolved by Texas Instruments.

If you have access to Mitel 8870 DTMF chips, you could use it in this circuit if you change the pins to match the corresponding functions. I have not done it, so I can’t help you.

Tone Decoder Schematic Diagram
Tone Decoder Schematic Diagram

I have not included a parts list, but I’ve tried to make the diagram self-explanatory. If you can understand it, you can probably build it.

Direct Connection
Direct Connection
Due to popular demand, I have included a modification that allows direct connection to the phone lines. I must say, however, that I have not tried the modification(I have been told it does work) and it’s probably against the laws of certain countries, states, provinces, townships, and telephone companies to connect home-made devices to the phone lines.

The software for this project is only a bare-bones program. The smart people that build this circuit will surely write some better programs and send them to me to include in this page. This program works fine, it’s just very basic. You must rename it with a .bas extension before loading it into a Basic Stamp.

DTMF Decoder Software for Basic Stamp 1