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vinodquilon

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  3. some weird effect of Telephone DTMF MT8870CE !!!

    in Projects Q/A

    Posted

    Look up the attachment .
    http://www.scribd.com/doc/27392525

    When power up the circuit all LED s 1 to 4 turned-on (I expect all turn-off).
    The problem is that, When I calling from remote end all the LED s blinking randomly for 1 sec (undesired result) and then 
    comes to a stable state.

    But this blinking effect doesn't exist when I manually turn-on the circuit after passing first ring.
    Now all LED s directly comes to the stable state without blinking as narrated below in the table.

    The surprise attack is that depending upon the service provider of telephoning remote person the
    stable state varies as indicated below:

    OPERATOR(India) LED4  LED3  LED2  LED1

    BSNL            OFF    ON    OFF    OFF

    VODAFONE        OFF    ON    OFF    ON

    IDEA            ON    OFF    ON    OFF

    RELIANCE        ON    OFF    OFF    ON

    AIRTEL          OFF    OFF    ON    OFF

    Local land-line end working on BSNL line.

    I doesn't believe in the supernatural effects, but there should be an evil influence.
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    I also tried the TOE in of DTMF IC, but after enabling DTMF Outs (after passing first ring) the
    first state would be the stable state described above without blinking problems. PWDN pin also not working.

    At a time subsequent to this stable state, DTMF IC obeys the normal truth table specified in the MITEL data sheet
    upon different key events on remote end after off-hooking local end.

    This is the third IC I have replaced, first two ones- MT8870CE,35P050C9,9004 AE
    didn't work at all. Third one- MT8870CE,12579.2,9125 AE- have the above issues. Next time I will choose KT3170 !!!
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    Without solving these issues I can implement my project by,
    1. Designing a ring counter which turn on the circuit only after passing first ring, thus no blinking problems at all.
    2. Designing a tri-state buffer with decoder at output of DTMF IC. Thus #<decimal> key combination can be designed.
      Circuit designed to turn on trisate for 5 sec when # key presses. During this interval I can press a decimal key,
      which is assigned to a particular house-hold device. Thus turn on-off corresponding devices. One of 16 outputs of
      Decoder IC will turn on at a time depends on DTMF BCD outs.

    First time I didn't notice this stable state changes  between different operators as
    in my workbench I have been using my personnel mobile phone as remote end and land line in my home as local end.
    When I switched to my friends' mobile having different connection I noticed this change. javascript:replaceText('%20;D',%20document.forms.postmodify.message);

    post-50016-14279144106456_thumb.png

  5. Efficient telephone audio (300-650Hz) fedback mechanism !!!

    in Projects Q/A

    Posted

    Can anybody suggest any efficient audio fed-back mechanism through telephone line
    so that the telephoning person can hear audio beeps, by which he can check the
    status of remotely controlled device (teleremote control).

    I have tried two circuits (see two attached files)
    http://www.scribd.com/doc/27346744
    http://www.scribd.com/doc/27346875

    But cannot hear any sounds at remote calling end. I have tried all frequencies in the range 300-650Hz by varying
    POT in the 555 astable audio beeps generator. But no result.

    I have selected this range as in this range there is no possibility for any DTMF tones.
    Thus audio frequency correctly fed-back to telephone line without detected by DTMF IC.

    Audio beep relay turns on only after off-hook relay get activated.

    I know that, in normal telephone line our vocal audio is captured by Mic, in turn converted
    into AF electrical signals. To check further fed-back mechanism ( & to imitate in my project),
    I opened my telephone set. But nothing can find out on a small, but complex circuit board
    inside the telephone set.  

    Does Anyone knows any circuits (that should be of workable) ???

    I forgot to say, I am feeding square pulses of audio frequency.

    fedback1.pdf

    fedback211.pdf

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