Phone audio interface


I needed a way to extract audio from a phone line for my DTMF decoder. I found a site full of examples on how to do it, and I built an interface that suited my needs:
  • RCA output, with a signal strong enough to be fed directly to an audio amplifier or line-in
  • The interface can stay connected on the line at anytime (no problems with the ring signal), and does not take of the hook
  • The interface must provide isolation from the line
  • I discovered that it can also be used to inject audio in the line


I found many examples on this site:
Telephone line audio interface circuits
The schematic of the phone audio interface I built:

TIP   o------)(--+    +-----+----+------ audio out
                 | || |     |    |
                  )||(      |    | 1n4001
                  )||(     _|_  _|_
                  )||(     /_\  \_/
                  )||(      |   -|-
                 | || |     |    |
RING  o----/\/\/-+    +-----+----+------ audio gnd
          600ohms insulation transformer
I have added a switch to choose between a 620 ohms and 33k resistor. This attenuates the signal coming out from the interface, provided there is at least a small load(10k works fine) between audio out and audio gnd.

Important: This circuit works in Canda and probably in the United States. It has not been tested in europe, but it may work if the phone system is similar enough.


Here are 2 pictures showing the outside and inside of my interface:

All the components I used are from an old answering machine, except the case, which I bought at King George electronic.


I'm not responsible if you use this to invade privacy of people.
I cannot be held responsible for any damages that could occur to you or your equipment while following the procedures present on this page. Also, I GIVE ABSOLUTELY NO WARRANTY on the correctness and usability of the informations on this page. Please note, however, that the procedures above have worked in my case without any damages or problems.

Now you cannot say that I did not warn you :)