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Phone line interface: Replacing a 1:1 transformer with an optocoupler

Discussion in 'Electronic Design' started by OBones, Apr 11, 2007.

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  1. OBones

    OBones Guest

    Hi all,

    I'm designing a system that uses a MT8870 DTMF decoder to detect tones
    on a phone line and the typical phone line interface is done via a 1:1
    600ohm "telephony" transformer, like this :

    -. ,--------o------ To MT8870
    Line )|( |
    )|( z
    -' '-. A
    | |
    Gnd |

    The two zeners are here to ensure the voltage does not go above Vcc
    applied to MT8870.
    My problem is that the transformers for these applications are bulky and
    take a lot of space. Hence I was wondering if I could replace the
    transformer by a diode bridge and optocoupler pair like this:

    ___ .----------. |
    +-+----|___|---|-. .-|-'
    A A | | |/ |
    Line -+ | | V -> | |
    -(-+ | - |> |
    A A | | | |
    +-+------------|-' '-|-- To MT8870

    My main concern is the frequency that the optocoupler can support. I
    have a bunch of 4N25 here and considering the max rise time and fall
    time with a security margin, I'd say it can transfer signals up to at
    least 30kHz which to me is enough for this application as DTMF signals
    only go up to 2kHz.
    Can anyone confirm that my calculations are right, and that the
    arrangement of the transistor side of the 4N25 is fine?

    Thanks a lot for your help.
  2. Ross Herbert

    Ross Herbert Guest

    Provided that you don't have any DC current flowing in a 600:600
    transformer they don't need to be big and clumsy. Since you say you
    only want to "detect dtmf" then you should be able to arrange your
    transformer with a blocking capacitor in series with the primary
    across the line.

    A 220nF 250Vdc film cap and a Tamura MG-21 (600:600) miniature audio
    transformer which has a 3dB freq range from 100Hz - 100kHz would be
    suitable. The transformer measures only 8mmx10mmx11mm.

    I dug up a circuit for a dtmf monitor here and it uses a 100nF cap in
    series with primary across the line.
  3. mpm

    mpm Guest

    Oliver - Two things:
    First, is the MT8870 still a valid part?? I thought these things were
    dinosaurs!! (Hard to find, not recommended for new designs, etc...)
    Unless this is a one-off design? I could be wrong though.

    Next, How do I use the text above? (The text that describes your
    Is there some program that interprets this becuase I could not make
    sense of it, even when I tried to re-print it in Courier font, etc..
    This issue arises from time to time and I am unable to help people out
    because I can't figure out their circuit design.

    Oh, one more thing: CML Microcircuits ( is a good
    source for all things "IC Telecom related". The CMX605 Digital to
    Analog (POTS) Signalling Interface with DTMF Encoder/Decoder might be
    just the thing for you. (In case the MT8870 really has gone belly-
    I don't recall if the 605 requires a transformer for interfacing..? I
    don't think so??

    mt8870 - $1.50 each
    I sure didn't have any luck with cmlmicro. I wanted some (c)mx614 modem
    chips once and tried sampling, no response at all. I tried buying, no luck
    there either. Do you know where to get their stuff in small quantities?
  5. OBones

    OBones Guest

    This is a one off, or a two off if I fry the first one ;-)
    And that circuit is easy to find around here, for hobbyists that is.

    Nothing much, it should just be pure ASCII text. I can see it just fine,
    and others can as well.

    Well, it better have some sort of protection as the phone line is about
    100 V when ringing and 48 when talking, while the circuit usually is 5V
    only. On top of that, I'd rather have some sort of isolation if
    lightning goes into the line...
  6. OBones

    OBones Guest

    Well, a regular phone line has a DC component, if I'm correct.

    I trust you on this, but how do I access this for a one off series, when
    I generally take the parts from the local hobbyist shop?
    I'd rather use the optocoupler isolation as it uses readily available
    parts that even come up in SMT packages, which are definitely smaller
    and easier on the layout than a through hole part.

    Thanks for that.
  7. I have used optocoupler for audio from phone lines.
    Firts quastion you must ask then is : Do you have a second isolated supply?
    In my case I had thta, so I could drive the LED of the opto cooupler
    with an opamp current source (linearity).
    Just picked the voltage of the line.

    Uin opamp
    -||-- +
    out ---------
    --- - |
    | \ / / LED
    | ---
    [ ]

    In case a switchmode is present the extra suply is just some extra turns on
    a potcore.

    At the reception side (different opamp!!) I had the photo transistor
    drive into the inverting input ... low impedance, good high frequency response,
    good linearity.

    In the example the OP (you?) gave the LED already draws quite a bit of current.
    I am not sure about the linearity of that circuit.
  8. It does, but you don't have to pass DC thru the transformer to get audio off
    the line. In fact, this is a good way to monitor a line without placing
    much load on it.

    Of course this all relies on another phone (or something) to allow DC to
    pass on the line to bring the phone line to off hook status. There's allot
    of decent information about different ways to interface to the line right
    Let us know how that works out, I'm very interested. I've never seen it
    done that way, but it may work fine. You're going to have to place a pretty
    severe load on the line to do it though IMO. I prefer the DC blocked
    transformer with clamping diodes myself. Watch out for those ringing
  9. If you'd asked in the right place, instead of adding to the clutter of
    this cesspool, you'd get a very simple answer.

    Pull a junk modem out of a junk computer or off a shelf somewhere, and
    use the parts from it to make the coupler. They all have the needed
    transformer and they are in no way bulky. The rest of the components
    right down to any needed optocouplers would be there too.

    Cordless phones would also offer up a suitable transformer in
    their base unit.

    Likely answering machines, too.

  10. Don Bowey

    Don Bowey Guest

    Not very "pure."

    For some reason your schematic's spaces appear as question marks (?).

    Does this look better? I did not fix the bottom line, but left it as it
  11. Ross Herbert

    Ross Herbert Guest

    Of course... However, since you are only "detecting dtmf" you don't
    need to pass the DC current on the line through the transformer
    primary. You will simply be "sniffing" the dtmf which is ac signal
    level so that is why you couple the primary via a cap.

    Unless you are also carrying out line looping (on-hold) functions on
    the telco side of the transformer, then you don't need a large
    transformer. The reason the ones you have seen/used are so large is
    because they have to handle a fairly large dc current in the primary
    because they are intended for line loop and hold functions on the
    primary side.
    Mouser have the Tamura MET-31
    (US market) and is identical to the Japanese MG-21 except for case
    color. They want a hefty price though ($13 per 1). I can post you an
    MG-21 if you don't have any success with your opto experiment.
  12. OBones

    OBones Guest

    Hum... I consider this a "design" question, hence the reason I put it in But if you have a better idea, please let me know.
  13. OBones

    OBones Guest

    I saw the question marks in mpm answer as well, but I don't know where
    they come from, I only see blank spaces in my original message.
    And I used AACircuit to draw it, so I'm quite puzzled.
  14. OBones

    OBones Guest

    Indeed. And I'm only sniffing the dtmf as there is a regular phone
    handset on the line that will actually put the signal there.

    Mind you, by fairly large, I'm talking about 28mm square and 15mm height.

    Thing is, I'm not residing in the US, but I will definitely go ahead
    with the opto experiment.
  15. OBones

    OBones Guest

    There is a phone on the line, yes, so I'm only monitoring what the phone
    is putting on the line.

    Well, I'm already using optos to detect line usage and ring "trains", my
    only interrogation is on the bandwidth that the optos can let through.
  16. mpm

    mpm Guest

    It must be something screwy on this computer, I guess?
    (I have no idea where to even start looking - XP strikes again!!)

    What is "AACircuit"? Shareware?

  17. OBones

    OBones Guest

    Andy's Ascii-Circuit

    Website is in German, but the software is in English. Very nice to have
    it when drawing schematics to be displayed in text only forums.
  18. Just ignore him, it was a design question. He runs around playing net-kop
    insulting people, encourages the and a.u.k. trolls, and then
    complains that it's a cesspool in here. Can anyone say hypocrisy.
  19. jasen

    jasen Guest

    should work.

    If you put the resistor before the bridge you can use 1N914
    diodes for the bridge.

    but ringing voltage can be over 100V (peak) andf if you shunt it you may
    stop other devices from ringing.

    it may be best to keep the bridge and put some sort
    of current-limit in series with the resistor.

    While we're discussing different approaches, how about a line-powered
    blocking oscilator built on one side of an ethernet transformer
    and an AM (or fm?) detector on the other side :)

  20. jasen

    jasen Guest

    yeah... some modems will even decode DTMF if you send the right commands,
    the only modification needed would be a series capacitor to stop it from
    seizing the line,

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