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On-off switch

Discussion in 'Electronic Components' started by Jim, Jun 17, 2007.

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  1. Jim

    Jim Guest

    I am in the UK.

    Can I build a simple device which would switch a pair of contacts on/off
    when my landline phone was being used.

    Am thinking of something like this. There might be a simple reed switch
    (do such things still exist) which would close its contacts if there was
    a current on the phone line.

    Perhaps I might need to improve the situation and wind the landline (or
    maybe just one of the two wires) around the reed switch.

    Would something like this work?

    Or can I buy a simple plug-in device for something like a fiver which
    closes its contacts when the phone line is active?
  2. Palindrome

    Palindrome Guest

    There are many designs on the internet for such a switch, eg:

    If you want a description of how it works, ask.

    AAstra Telecom ( /apparently/ makes the LumiNET(tm)
    line-in-use indicator.
  3. Stevie Boy

    Stevie Boy Guest

    ">I am in the UK.
    While I'm not to sure what your hoping to achieve but you can buy a priority
    (can be called by another name) switch which basically is a 2 outlet
    telephone adaptor plug for less than a fiver that isolates the other
    connected line when in use. This is achieved from a array of transistors
    inside the unit which consumes it's power from the line.
    This may be what your looking for.

  4. Graham

    Graham Guest

    As Sue suggests go for a voltage sensor solution rather than
    a current one. On-hook a phone line has about 48v across
    the wires, Off hook it falls to about 9v so anything below
    (say) 20v can be assumed to be off-hook.

    You might want to seek out a line interface that has the
    necessary approvals, especially if you are working on
    somebody else's line.
  5. Lurch

    Lurch Guest


  6. On Web

    On Web Guest

    What would you use this for?

    At the moment I can only think of the obvious application: bugging other
    peoples conversations, something I'd not be happy to assist with..
  7. Owain

    Owain Guest

    Privacy adapter.

    Quite a lot of them come with LEDs showing the line is in use. An LED is
    one half of an optoisolator.

  8. Owain

    Owain Guest

    Automatic switching of a recording device is one possibility, although
    it is (in the UK) perfectly legal to record one's own calls.

    It might also be useful to mute a radio or other sound source
    automatically when answering the phone.

  9. Lemmo

    Lemmo Guest

    Firstly, your mind is too narrow:

    (1) The desklight in the study would switch on and I could then use a

    (2) A strobe/flash is triggered so those who can not hear the phone
    above a loud stereo can tell it has run.

    (3) etc


    Secondly, your mind is poorly uninformed:

    "Recording and monitoring telephone calls" (Oftel/Ofcom)

  10. mc

    mc Guest

    There are 2 basic approaches.

    One is to sense current flowing through the line.

    The other, probably easier, is to sense the voltage across it. This will be
    well above 30 volts when the line is not in use, well below that when it is
    in use.

    Your sensing circuit needs a very high input impedance (like 10 or 20
    megohms) to avoid putting a measurable load on the line.
  11. mc

    mc Guest

    What would you use this for?
    Good point, but he doesn't have to ask us for technical help for that --
    there are commercial gadgets available. At one time Radio Shack sold one
    (in the USA).
  12. Beachcomber

    Beachcomber Guest

    If you want to save time doing the research, Viking Electronics
    appears to have an off-the-shelf solution.

    Relay provides contact closure on ring as well as loop.

    (no affiliation) just found this when I had a similar need for it.
  13. On Web

    On Web Guest

    Very heath-robinson.

    Well I do wonder what the OP is up to since they don't say..
    I have a phone line, but I doubt the other house occupants would be pleased
    if I rigged up some kind of device to start recording in my absence.
    Of course, we have no idea what the OP wants to do, but I can certainly
    imagine uses that are unsavoury (I'm not suggesting that is the case).

    I wouldn't help anyone along with such a device without knowing the end-use,
    but clearly that's just me. It also seems sensible to mention the actual
    usage since there may be a ready made/alternative solution besides jury
    rigging the phone line.
  14. On Web

    On Web Guest

    Jim is also trying to capture the phone numbers of people calling the line,
    so maybe that's the application..
  15. ian field

    ian field Guest

    A few years ago Elektor magazine published a constructional project for a
    reed relay based self energising latch for dial up internet intended to
    defeat internet rogue diallers that break the connection you dialled
    yourself and redial a premium rate number.

    The circuit consisted of a reed unit with an energising coil round it, wired
    in series with the reed, a push button is in parallel with the reed so the
    connection is made while dialling, once the hookswitch is closed and current
    flows through the coil the reed pulls in and the button can be released. For
    a rogue dialler to dial it must first drop the existing connection - which
    also drops out the reed making the dial out impossible.

    Since this demonstrates that it is possible to wind a coil round a reed
    element that can be operated by the telephone line current, it might help if
    you can obtain a copy of the article for the coil winding details and modify
    the circuit for your use.
  16. Franc Zabkar

    Franc Zabkar Guest

    My immediate thought was that a line in-use indicator could tell you
    if someone was talking on the phone before you tried to send a fax or
    dialled in to your ISP. Conversely, it could also be used to indicate
    that a dialup Internet session was in progress.

    - Franc Zabkar
  17. John G

    John G Guest

    There are about 1200 entries in Google about "Off Hook indicator" for

    All of which are most likely illegal in some jurisdictions.
  18. I only see one post from him, the first one:

    There is no claim of any attempt to capture callers' numbers there.

    Regarding issues of recording, deriving switch signals, this is legal in
    the UK, you just have to watch how the law applies in specific instances.

    You talk of 'jury rigging' a phone line, which is as loaded a phrase as I
    ever heard, it implies some kind of illicit activity. Any real jury knows
    that a person is guilty until proven innocent.

    If you want to be cautious, just point the OP to a page that discusses the
    regulations governing user connections to the user side of the master
    socket wiring, and the laws governing lawful making and use of
    recordings. The rest you must leave to their discretion, you can't police
    their morality.

    The easiest course is to look at what is commercially available in a shop.
    If it's on open sale, it is safe to assume that discussing its function is
    not only legal, but wise, and it might be cheaper to make than to buy.
    Doubtful though, unless you buy from Maplin or other shop that charges
    around twice what many shops accept as reasonable.
  19. Indeed. It can actually help with privacy. Without it, what's to stop
    someone picking up the line and finding themselves a willing or surprised
    eavesdropper, even against their better judgement. Such a device is as
    innocuous as an engaged sign on a toilet door. Many people would encourage
    its use, that way at least it takes deliberation, with less excuse for
    'accident' in shared households if a person picks up a handset to listen.
  20. And incidentally, a caller ID display device is legal in the UK,
    commercially available, often used by people who need to know in advance if
    it is safe to answer the phone.
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