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Modernise old rotary telephone

Discussion in 'General Electronics Discussion' started by Myjyc, Aug 3, 2015.

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  1. Myjyc

    Myjyc

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    Aug 3, 2015
    I would like to transform an old rotary telephone using an arduino micro + MT8880 to dial numbers coming either from the USB interface (serial), to dial straight from the computer, or from a number memorised with a shortcut dialed on the rotary dialer, or even from a number dialed on the rotary dialer. I would isolate the rotary dialer from the tel line, and read it straight with the arduino. The rest of the telephone stays same, just one button to tell the arduino to dial (when using the rotary dialer). The problem is to insert the tones from the MT8880 to the line; I guess I can not add a Transformator in serie or parallel to the actual one, can I? I thought of replacing with a relay the microphone with the audio from the MT8880 only when dialing, would this work? (note that there is only 3 wires coming from the micro and speaker, they are connected to the transformator with one common to both)
    Any one having an idea?

    Thanks!
     
  2. Martaine2005

    Martaine2005

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    May 12, 2015
    Hi Myjyc,
    I know nothing about Arduinos or programming.
    But the old rotary telephones didn't work on tones. That all happened with the newer push button phones.
    The rotary was electrical pulses on the return, not turning the dial to a specific number.

    So you would have to find a way of turning the dial to a number, and letting the dial freewheel back to generate the pulses.

    Martin
     
  3. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Uh yeah, the rotary phone could be considered a primitive form of digital dialing. I haven't tested my line in over a year but when I did it still accepted it!

    Chris
     
  4. Tha fios agaibh

    Tha fios agaibh

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    Aug 11, 2014
    I'm not an Arduino guy either, but I imagine the click signal from the original dialer mechanism could be fed to an input that a microcontroller would then register (count clicks) and then convert the count to the equivalent touch tone (oscillator ckt) for the equivalent number. I'd imagine there would be a delay between when you dial, and the actual call being placed?
     
  5. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    I hate to be a killjoy but I don't think this is a practical project.

    Chris
     
  6. Tha fios agaibh

    Tha fios agaibh

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    Aug 11, 2014
    I for one would not pesue it.
    But if the op has the determination, It's certainly possible.
     
    Last edited: Aug 4, 2015
  7. Tha fios agaibh

    Tha fios agaibh

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    Aug 11, 2014
    Of course this rotary phone would not look original with a power cord or usb cable hanging from it. Lol
     
  8. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    I don't know anything about Arduinos other than they're a family of uCs. I would think that they can generate DTMF or Pulse Dial. That being the case I don't see much point in automating the mechanical end of the rotary phone. Why not just use it to just talk and listen? Let the Arduino do the auto dialing. Like I said, as of a year ago I could still pulse dial here. My bet is I still can.

    Chris
     
  9. AnalogKid

    AnalogKid

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    Jun 10, 2015
    Both the arduino and PIC have trouble doing touch tones well because servicing the D/A and its update rate suck up so much CPU time. The 8880 is overkill for the project, but should work. 8880 app notes have interface circuits. Also, search for "telephone hybrid schematic" to find many more.

    ak
     
    Tha fios agaibh likes this.
  10. Tha fios agaibh

    Tha fios agaibh

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    Aug 11, 2014
    I expect he wants to do it just for the novelty of it. And some of us on this site are a little geeky with how we spend time. Lol

    Back in the late 80s I used to have a wrist watch that would dial the phone by repeating the touch tone stored in memory. You just held the mic over your wrist and it would dial for you.
     
  11. Martaine2005

    Martaine2005

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    May 12, 2015
    Haha, you been phone phreaking??
    Are you related to Steve Wozniac?

    Martin
     
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  12. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Ha, the other day I was digging through my junk boxes and found a old Radio Shack Auto-Dialer given to me by a friend many years ago. Like your watch it's placed next to the phone's mic. I think this thing stored multiple contact numbers.

    Pics & Arduinos probably do have the issues stated but they should easily handle Pulse Dialing. For me to test whether my line still accepts it I'd have to dig out (another of many boxes) one of my old modems. Do that or find one of my old rotary phones. Never mind that. It would tantamount to extremely DEEP DRILLING! :)

    As for DTMF: There are plenty of DTFM encoder/decoder chips available that can easily be controlled with a uC.

    Chris
     
  13. Myjyc

    Myjyc

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    Aug 3, 2015
    Thanks, I have seen all those "telephone hybrid schematic" but do you think i can put that in serial or parallel to the existing microphone and speaker transformer that i want to keep?
     
  14. Gryd3

    Gryd3

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    Jun 25, 2014
    A little info for everyone.
    Arduino is a product which most often uses the AVR microcontroller. It has a pre-built library of functions, and a special bootloader pre-flashed on the AVR that allow for rapid design and programming.
    As mentioned above, DTMF can chew up a large number of resources... and although it can be done, I think you should look into something like a TP5089 to generate the tones for you. Or look into DTMF transceivers.
    As far as 'is it worth it'? Depends. If you want to do it, go ahead, but be careful when dealing with the telcomm line... There is a constant DC voltage as well as an AC voltage during ring and conversation. You either need to get a dollar store touch-tone phone for parts, or do some serious research to ensure you properly interface with the telcomm lines.

    I certainly vote for getting the cheapest touch-tone phone you can buy, and taking it apart. The arduino can interface with the circuit and provide the much needed bridge between the pulses and the tones.
     
    Last edited: Aug 5, 2015
    Tha fios agaibh likes this.
  15. AnalogKid

    AnalogKid

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    Jun 10, 2015
    Good idea. Fortunately, the MT8880 *is* a DTMF transceiver.

    ak
     
  16. Myjyc

    Myjyc

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    Aug 3, 2015
    Thanks Gryd3, I'll check the TP5098, maybe I power it with the telephone line and put an isolator between this and the arduino.
     
  17. Gryd3

    Gryd3

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    Jun 25, 2014
    Be careful with this...
    The telcomm companies do not put voltage on the line to provide 'power' for their customers devices. If you draw too much, or interfere with the line you can get into trouble ;)
    That's part of the reason I mentioned gutting a cheap touch-tone phone from the dollar-store.
    Of course, if you do your research and build it properly from scratch, you should be fine.
     
  18. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    FYI, I just stumbled upon youtube vids regarding this topic. Evidently there are commercially made inexpensive modules that convert the pulse dialer to DTMF.

    Chris
     
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  19. Myjyc

    Myjyc

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    Aug 3, 2015
    I have a VoIP modem, so only the modem could complain ;-)
     
    Gryd3 likes this.
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