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DTMF to telephone line interfafce

Discussion in 'Electronic Design' started by eeh, Oct 30, 2005.

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  1. eeh

    eeh Guest


    Could anyone show me an example circuit which demostrates how to
    interface the DTMF signal from MT8880 or CM8880 to the telephone lines?
    I have found in the internet but cannot find a match.

  2. Pooh Bear

    Pooh Bear Guest

    Can I suggest you search for *application notes*.

    Most semi manufacturers supply these to show how best to implement their

    Who makes the above for example ?

  3. OBones

    OBones Guest

    just use a 1:1 600ohms transformer and that's it.
  4. That's one of the things I suggested when he posted the same question
    in another newsgroup in the past week.

    The first suggestion was that he buy a cheap phone, and use that
    intact. It's obviously got the interface in place, and you get the
    touch-tone encoder too.

    Note also that in the other newsgroup, people are still wondering what
    the purpose of this project is. Because a solution obviously comes
    from the situation. If it's something like he has a rotary phone but
    needs touch-tones to work with automated systems at the other end,
    there's the time honored scheme of feeding the touch-tones into
    a small amplifier and then speaker, and then holding the speaker
    up to the microphone of the telephone. Solutions are a dime a dozen,
    but none of them may apply until he reveals exactly what he's trying
    to do.

  5. Jim Thompson

    Jim Thompson Guest

    Years ago, like late '80's, early '90's, I was doing a project at

    They had some kind of digital phone system that wouldn't send DTMF,
    plus they forbade long distance calls (but not 800 number calls)
    without going thru the company operator with a project number to
    charge to. Thus I couldn't call other clients.

    I subscribed to (IIRC) Sprint, which had an 800 number you called,
    then keyed in the number you wanted to call.

    I found a device at Radio Shack that fit in a shirt pocket. You held
    it up to the phone and dialed with the buttons on the device.

    I don't know if it's still made or not. I still have mine around here
    somewhere... I recall seeing it during an enforced office cleaning ;-)

    I'll try to find it and get the RS part number.

    Found it!

    33 Memory Pocket Tone Dialer, Model 43-141, length of a pack of
    cigarettes, but narrower and thinner, 3 AAA cells.

    ...Jim Thompson
  6. Pooh Bear

    Pooh Bear Guest

    You have to be careful with loading the line though.

    Might also want to do an 'electronic hybrid' to stop the injected signal
    passing back into the receiver.

  7. Jim Thompson

    Jim Thompson Guest

    ...Jim Thompson
  8. Don Bowey

    Don Bowey Guest

    As I read the datasheet, the dtmf output of the IC is -2 dBm, but into a 10k
    Ohm load, so you will need a matching transformer (10k:600-1300 or so). Or
    (best) you can use some other transformer with an op-amp to get the right
    level and a decent nominal impedance.

    Exactly what else you need to connect the transformer to the Tel Line,
    depends on what you are attempting to do.

    I assume you have looked at the datasheet so you know how to build the final
    circuit using the 8880, right?

    I got 163 google hits shearching for "dtmf dialer using a MT8880."

  9. Bob

    Bob Guest

    Yep. It's not as easy as "just use and that's it".

    I suspect that 'eeh', the OP, is some sort of spy, terrorist, or alien
    creature. That's why he/she won't tell us what he/she is doing.

    I remember, back in the days when I, too, was an alien -- building blue
    boxes and black boxes. *That's* how you learn about the phone system.
    Ahhh....the memories...

  10. Bob

    Bob Guest

    Specifying an output level into a given load does not imply its output
    impedance. Also, if you used a 10K->600ohm transformer you would get an
    EXTREMELY low level at the 600ohm side, because of the large--to-small turns
    ratio of that transformer.

    Now you're talkin'. However, I suspect that googling is beyond the OP's

  11. Pooh Bear

    Pooh Bear Guest

    Why ? You're doing that backward thinking thing about dBs again aren't you ?

    The IC will work into any load that isn't below the rated value.

  12. Don Bowey

    Don Bowey Guest

    When output is spec'd at 10kOhms I'm certain it is not going to be the same
    output level at 1000 Ohms. What was your point?
  13. Don Bowey

    Don Bowey Guest

    It's output is rated at 10kOhms. The telephone line Z will probably be in
    the range of 900 to 1300 Ohms.

    So what's your point?
  14. Pooh Bear

    Pooh Bear Guest

    You buffer the output via an op-amp for a few pence you dimwit ! ( see my note
    about the 'electronic hybrid' ). There are many op-amps that have no trouble
    driving telephone line impedances.

    If ppl stuck transformers everwhere you suggested we wouldn't be able to move for
    iron !

    Also - as was noted by others - a transformer will reduce the signal voltage !

  15. Bob

    Bob Guest

    If the output is spec'd into a 10k load then this is what the manufacturer
    chose to rate its test load at. This DOES NOT mean that its output impedance
    is 10kohms. It's important to make that distinction since they are separate

    Typically, an output has a driving impedance of a few ohms. A manufacturer
    will specify its minimum load impedance so as to limit the amount of power
    that the output needs to, or can, drive. This is independent from its output

    You should experiment with devices similar to this. Any old opamp (with
    appropriate hookup) will do. What you'll find is that whether you connect
    its output to a 10k or 1000 or 100 ohm load, the output level (for any given
    input level) will not change much. In fact, in terms of dB's, it will
    probably decrease less than 0.1dB from a 10k to a 100ohm load. Try it.
    You'll get a better feel for it.

  16. Don Bowey

    Don Bowey Guest

    1. You still did not make any new point that I can see - just your usual
    "jump on a post and add confusion bullshit." I noted in my original post
    that a best approach might be to use an op-amp and a transformer. However,
    since we don't know what the OP is doing, one should not conclude that gain
    IS NEEDED. I estimate the attenuation from the 10k to 1k connection would
    be about 18dB, which leaves the signal within working range of the CO under
    average conditions, though not within the Standards requirement. The time
    to work on a real design is after all the OP's facts are known.

    However, it would be good of you to deliver a good design to S.E.D so
    everyone could see your transformerless approach.

    More BS nonsence on your part. The connection can ONLY be transformerless
    if his gadget uses batteries and is isolated from earth ground. And why did
    you recommend using a hybrid transformer? The OP said nothing about wanting
    to receive dtmf signals.

    Well, Duh! Of course it will, but unless the transformer is a piece of
    junk, it's attenuation is not going to be a killer.
  17. Don Bowey

    Don Bowey Guest

    I am well aware of this, but the device is not just an op-amp, and it would
    be foolish to commit a design based on an assumption that it's output is
    just like an op-amp.
    You speculate too much.
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