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Device to show when phone is in use

Discussion in 'Electronic Design' started by Andy Petro, Mar 27, 2006.

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  1. Andy Petro

    Andy Petro Guest

    Bell used to sell a device that told you when a phone was connected.
    You placed it in series with the line and a light would indicate when
    the line was busy. Any idea as to how to build one as they are no
    longer sold.
     
  2. Ken Taylor

    Ken Taylor Guest

    Search on "telephone in use indicator". Plenty of circuits and ready-mades.

    Ken
     
  3. JeffM

    JeffM Guest

  4. ****** Warning Some Countries forbid connection of non-government approved
    telephone equipment OR tampering/tinkering with telephone line ****

    *** If info is for educational purposes only ******


    1st Understand the concept of phone not in use and hung-up.

    How do you detect a phone is off hook.... what happens to the electrical
    signals? Ask yourself those questions.



    When the phone is on-hook (hung up) the phone has a higher DC resistance...
    as if it were not connected....

    When the phone is picked up a lower DC resistance is applied by the hook
    switch to the phone line.

    The lower DC resistance has 2 effects

    1. Draws current from the phone exchange typically 20mA to 90mA depending
    on how far from the exchange

    2. The voltage drops from typically 48 volts to typically 6v ~ 18v
    depending on how far you are away from the exchange.


    OK.... you how have two effects you can measure....

    current ... which requires a series in-line device

    voltage.... measure the voltage across the phone point.... parallel circuit.




    NOW.... when you measure electrical signals you should not effect the item
    you are measuring.

    So a series current or parallel voltage measurement is required.


    Note... there are other complex factors involved..... but this info may
    partly explain the DC characteristic of phone and exchanges.


    JG
     
  5. Fred Bloggs

    Fred Bloggs Guest

    The indicator circuit is required to present an ultra-high impedance to
    the line when on hook. This little flasher from Tech-Lib does the job:
    View in a fixed-width font such as Courier.

    ..
    ..
    .. ~~
    .. +-------+------+|<|-[180]-+
    .. FWB | | | + |
    .. +-------+ | | | -----
    .. TIP ---|~ +|--------+ [1M] [1M] | 9V ---
    .. | | | | | | Batt |
    .. | -|>|- | [22M] | | 1U | |
    .. | | | +-------|-------+--||--|----------+
    .. | | 0.01U| | | | + | |
    .. RING---|~ -|--+-||--+ | | | | |
    .. +-------+ | | | | e | |
    .. | [22M] c | |/ | |
    .. | | |/ +-----|2N4403 | |
    .. | +--|MPSA-18 | |\ | |
    .. | | |\ | c | |
    .. | | e | | c |
    .. | | | | | |/ |
    .. | [1M] | [2.2M] +----|2N4401 |
    .. | | | | |\ |
    .. | | | | e |
    .. | | | | | |
    .. +-----+----+-------+--------------+----------+
    ..
    ..
     
  6. Ian Stirling

    Ian Stirling Guest

    http://cgi.ebay.com/PHONE-TAP-DETEC...ryZ73336QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
    Was found in several seconds of searching.
    Minus the bullshit about phone tapping, looks like it's the right
    device.
    Presumably there are cheaper ones.
     
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