Connecting an Arduino to a telephone line?

Question

I would like to connect my Arduino Uno to a telephone line in order to answer a call, record a sequence of DTMF tones, generate a DTMF tone in response, and hang up.

The telephone line uses a standard RJ11 connector. When a touch-tone phone is connected to the line, it receives one of two rings:

• a standard ring, which signals an incoming call
• a shorter "two-ring" sequence which signals a call from a local intercom

I would like my Arduino to listen for the "two-ring" sequence and only answer those calls.

How would I go about connecting my Arduino to the telephone line? Is there a shield designed for this purpose? I know I can use the tone() function to generate the DTMF tones, but I'm assuming I would need to perform some form of Discrete Fourier transformation in order to "read" the DMTF tones from the other end of the line.

Any suggestions would be appreciated.

Answer 1

You can interface arduino to telephone line using MT8870D chip. This IC can decode DTMF tones. Below is a reference schematic from the datasheet.

Another popular design floating on the internet is below:

There are pre-build modules available from futurelec :

You can use Holtek HT9200 IC to encode DTMF that can be interfaced with Arduino. Below is the reference schematic from its datasheet.

Answer 2

You'll need some hardware assistance for the DTMF detection and generation; I doubt many Arduinos (you didn't say which, but maybe ARM?) have the horsepower to run an FFT (and deliver the results while anyone still cares....)* tone()may not be precise enough in frequency for DTMF detection by the calling device; you'll have to check that out. A quick search finds a number of articles on building or buying tone detectors and generators.

Ringing pattern detection is a little easier but you'll still probably want to sense the ringing and convert it to a level in hardware, rather than tracking the instantaneous line voltage and trying to distinguish in software between noise, ringing, noisy ringing, and various degrees of quiet.

_{* But maybe this Hackaday project can give you some alternate and computationally feasible ideas.}