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Telephone "in use" indicator

Discussion in 'Electronic Basics' started by Earl, Jan 15, 2004.

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  1. Earl

    Earl Guest

    I want to build a telephone in-use indicator from a diagram
    I found on an electronics website. In the diagram two wires
    from the telephone are connected to the circuit. However,
    I clearly remember that telephone cables have more than just
    2 wires inside them. How would I know which ones to use?
     
  2. I don't know about the UK, but in the US we use the red and green
    wires for a single line telephone. The yellow and black are for a second
    line, or on very old "princess" telephones that had low voltage AC to
    power the lighted dial.

    Newer, three pair, phone wire uses the blue/white pair for line one.

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    Take a look at this little cutie! ;-)
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    Michael A. Terrell
    Central Florida
     
  3. CapMan

    CapMan Guest

    Some phone cables do have more but only 2 are needed. The tip and ring...
     
  4. Earl

    Earl Guest

    I don't know about the UK, but in the US we use the red and green
    I have just opned an old phone I had in the attick. It seems to have the
    following wires, with letters where they connect to the circuit board,
    as follows:

    E green
    b red
    s blue
    a white

    Strangely, the E is in upper case and the others are in low case
    any ideas what I need to use ?

    btw. it is a pulse phone, about 15 years old.
     
  5. cpemma

    cpemma Guest

    If you're in the UK I'd be wary of connecting a home-built 'net circuit to
    the line,

    (a) it's against BT terms & conditions - affect the line & you're in
    trouble,

    (b) the UK phone voltages are quite different to the US levels which your
    circuit may be designed for.

    If you must do it, at least find a UK-specific circuit.
     
  6. Earl

    Earl Guest

    (a) it's against BT terms & conditions - affect the line & you're in
    I'll give you the link and you can judge for yourself if this circuit is
    safe:

    http://ourworld.compuserve.com/homepages/Bill_Bowden/page12.htm#inuse.gif

    Regarding a. I suspect this exists in all countries. Regarding b. can you
    please
    describe the voltage differences, and suggest circuit modifications if
    needed.

    tia.
     
  7. cpemma

    cpemma Guest

    Once past the master socket, we use a 3-wire system with a "bell" wire, and
    48v DC between the other 2 (off-line), 5-10v (in use) which correspond to
    ring & tip, see http://www.diyha.co.uk/telephones/pots.html for wiring
    details. I've seen figures putting the ring signal at 80v AC

    I can't remember the US figures, but a google groups search should turn them
    up and perhaps a UK-specific circuit.
     
  8. Dbowey

    Dbowey Guest

    Earl posted:
    <<
    I'll give you the link and you can judge for yourself if this circuit is
    safe:

    http://ourworld.compuserve.com/homepages/Bill_Bowden/page12.htm#inuse.gif >>


    The DC Ringer Equivalent is about 4, so I would have problems with the circuit
    if used in the U.S. I'm not up-to-date on the UK requirements.

    I didn't calculate the leakage current from the DC voltage running the device,
    back to the tel. line, but in the U.S. absolutely NO voltage is permitted to be
    placed on the line from customer equipment (for loop-start lines, which this
    is).

    Don
     
  9. It's the green and red wires, or the white with a blue stripe and blue
    with a white stripe.

    Here's the URL of the schematic below of the indicator you *should*
    use. If you use one that doesn't have a battery but sucks power from
    the line itself, the telephone company may find the line tests bad
    when they do their periodic tests. If so, they may send out a
    repairman, and when they find it's your equipment causing the problem,
    they may bill you for the time. So don't use a line powered
    indicator.

    http://ourworld.compuserve.com/homepages/Bill_Bowden/page12.htm#inuse.
    gif

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  10. Earl

    Earl Guest

    It's the green and red wires, or the white with a blue stripe and blue
    i.e. E and b
    how does it work ? what is the reason for C=0.1uf
     
  11. How? Well, simplified, the transistor requires a voltage of less than
    about 3V to conduct and light the LED. The three resistor voltage
    divider puts more than 3V across the 680k when the phone is on hook,
    and the voltage drops below 3V when the phone is off hook, and the LED
    lights up.

    You could also look at it as a current summing point at the junction
    of the transistor's base and the 680k. When the current from the
    phone line is equal to or greater than the current thru the 680k, the
    base sees no forward bias and the LED stays off. When the current is
    less than the current thru the 680k, that current from the 680k
    forward biases the transistor and the LED lights.

    And yes, you have to take into account the .6V drop across the base to
    emitter junction.

    The 0.1 uF is to filter out RFI or other noise, even ringing current,
    from the phone line, so all you see is the DC voltage.


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    ###Got a Question about ELECTRONICS? Check HERE First:###
    http://users.pandora.be/educypedia/electronics/databank.htm
    My email address is whitelisted. *All* email sent to it
    goes directly to the trash unless you add NOSPAM in the
    Subject: line with other stuff. alondra101 <at> hotmail.com
    Don't be ripped off by the big book dealers. Go to the URL
    that will give you a choice and save you money(up to half).
    http://www.everybookstore.com You'll be glad you did!
    Just when you thought you had all this figured out, the gov't
    changed it: http://physics.nist.gov/cuu/Units/binary.html
    @@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@
     
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