MAKE: Blog: HOW TO - Make a DTMF phone dialer

HOW TO - Make a DTMF phone dialer

Box 1
Here's how to make a fun DTMF Phone Dialer via Zedomax (source and schematics included). Alex writes...

The company I work for recently upgraded their phone system. They got a new IP phone system that can supposedly do everything - the new phones have big LCD screens with fancy menus, lots of different modes, PC connection for remote control of the phone, e-mailing voice messages, and tons of other features. All that said, there was a critical piece missing from the picture - a way to dial a stored number while already in a call. This is a feature I use often - I'm using a calling card to call home and I need to first dial the 800 number and then send my calling card pin number. Well, believe it or not, the cool new phones with all the fancy software couldn't do this and I had to switch back to pencil and paper! "We will include this feature in release 8.0 or possibly 8.5" was the response from the phone company. Yeah right, like I'm going to wait for 8.5!
So, here is the obvious solution - a dialer box that connects to the phone and can be used to store and dial phone numbers, pins, credit card numbers and what not. The circuit uses a NTE1690 DTMF dialer chip and a PIC16F690 microcontroller.

Because this is an IP phone and I cannot just send the DTMF tones over the line, the easiest place to plug in the box is between the phone and the handset. The box has two RJ9 ports at the back - one gets connected to the headset's port on the phone, the other to the headset itself. The dial tones are sent over the microphone line. This way, it works both on IP-phones, and on standard (old fashioned, non-IP) ones.

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Comments

Oldest comments listed first.

Posted by: meh on June 10, 2008 at 12:41 PM

couldn't you also use the PWM output of a uC to generate the DTMF rather than buying a specific (and perhaps unavailable in some markets) chip? like in AVR314 perhaps?

Posted by: The Oracle on June 10, 2008 at 3:16 PM

DTMF plays two tones at precise frequencies at the same time (it's what the acronym means). It would be extremely hard to generate the effect in software with the limited power on an 8-bit MCU.

Also, PWM is a matter of duty cycle. The frequency is not a major issue, and not controlled critically enough to do DTMF.

Anyway, very cool project. I played with this many years ago with the ST25089 tone generator chip. Afaik, that chip was discontinued, and I have a project in mind that needs one. The NTE1690 seems to be the NTE cross-reference of the 25089. Is that true, and is it actually easily available?

Posted by: Alex on June 11, 2008 at 5:42 PM

Now I found out why my site's traffic jumped that much in the last couple of days - the project has been linked to from makezine.com. :-)

About NTE1690: yes, it is a readily available chip, at least here in US. I got it from Fry's Electronics - they had it in stock. The 3.579545 MHz oscillator was not a problem too (I got mine from another local shop, HSC).

I guess it would be possible (although it might prove to be really complicated) to implement the DTMF in software, but I'm lazy and using NTE1690 was easier.

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