PC Audio Phone Dialer With Messaging

Discussion in 'The Projects Forum' started by scrappy, Jul 15, 2014.

  1. scrappy

    Thread Starter New Member

    Jul 15, 2014
    I'm using the attached circuit as a phone dialer and audio message deliverer. I have two issues that I need help with:

    01. When I use the circuit from my PC that has an amplified audio port it works fine. When I use it from my Raspberry Pi the volume is not loud enough, even at max, for it to dial. So, I need to build in an amplifier of some sort. I'm thinking of using an LM380 but I'm not sure which side of the circuit I should use it?
    A. On the PC audio out before the transformer?
    B. On the PC audio out after the transformer?
    C. On the phone line section after C1 and R1?
    D. Other?

    02. I would like to protect this circuit from accidental rings coming from the outside world. It is connected to an analog phone line that is not supposed to ring but it can if someone dials the number.

    FYI - R1 is a 470 ohm resistor that makes the circuit trigger an off-hook state. It has a relay to control it that I have not included in the schematic.

    Any other advice or suggestions would be greatly appreciated. Thanks.
  2. DickCappels


    Aug 21, 2008
    Definitely put he audio amplifier on the PC side of the transformer. Do as little as necessary to the phone line side.

    The ring voltage is current limited, so you can limit the voltage across your circuit by using two "back-to-back" zener diodes to limit the P-P voltage. Connect two zener diodes in series with their cathodes connected together. Connect the anodes to the transformer secondary windings.
    scrappy likes this.
  3. scrappy

    Thread Starter New Member

    Jul 15, 2014
    What value/voltage zener diode(s) would you recommend. I found a couple of examples on line but the voltages they recommend seem low to me? One example has two 2.7v .4W. I guess that means they will break down at higher than 2.7V and wouldn't that cause a short circuit? Sorry, I'm not versed in zener diodes at all.
  4. shteii01

    AAC Fanatic!

    Feb 19, 2010
    What about op amp in buffer configuration. That should block the incoming signals.
  5. Ramussons

    Active Member

    May 3, 2013
    Use an auxiliary contact of the relay (NO) to isolate the setup from the telephone line to block the Ringing voltages.

    scrappy and DickCappels like this.
  6. scrappy

    Thread Starter New Member

    Jul 15, 2014
    I used the audio amp and diodes as suggested by DickCappels, I'm also using the suggestion by Ramussons. Thank you! It works now. I'll upload a final schematic and outline of how it works so it will be a complete project others can use. :)
  7. scrappy

    Thread Starter New Member

    Jul 15, 2014
    I'm using the attach circuit. When the relay is triggered then the R1 470 resistor causes an off-hook to be detected by the phone system. I then use an audio player to play DTMF tones and a message with Linux. I'm using aplay. Here is an example.

    aplay dtmf_4.wav dtmf_4.wav dtmf_1.wav

    The above calls 411 for directory assistance.

    You can record any message and add it anywhere after the dtmf tones.

    aplay dtmf_9.wav dtmf_1.wav dtmf_1.wav fire.wav

    The above would call 911 and play a pre-recorded message that you currently have a fire situation. As you can see, I'm using this for a home built security system.

    I used Audacity, free digital audio editor, to create the dtmf tones for each number and the voice message/recording. I have built and tested this circuit and it seems to be working fine. Thanks.