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Need "electronic switch" for opening line??

Discussion in 'Electronic Basics' started by zerang shah, Sep 20, 2004.

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  1. zerang shah

    zerang shah Guest


    I have a circuit that connects to the phone line. As long as the
    circuit is connected, it seizes the line to get a dial tone. I added a
    manual toggle switch across the ring line so that I can leave the
    device connected and recieve calls, then switch it on when I need it.
    My circuit looks like this:

    RING(from telephone line) ---------/ -----------------------

    Everything here works fine, when I close the switch everything
    functions correctly, and when I open the switch I can still make and
    recieve phone calls.

    Now, instead of using a manual toggle switch, I'd like to be able to
    close the line electronically some how. I could use a transistor -
    except the voltage range of the telephone line is something like 48v,
    and I'd be opening/closing the switch with just 5v. I don't think any
    transistor can function under those tolerances.

    So here's what I need: A way to close and open the line electronically
    by the presence or lack of presence of +5v. Any ideas??
  2. scada

    scada Guest

    A 5VDC Relay driven by a NPN transistor, common emitter would give the
    needed isolation.
  3. Use a relay. You can get them that are activated by 5V.

    Bob Monsen
  4. (zerang shah) wrote ...
    This is not really an answer since others already answered you, but
    reminds me of a neat little trick for modifying phones in the house to
    keep people from easedropping on your calls and also to keep other
    extensions from throwing you off a modem by people picking them up.

    If I recall correctly (this is all off the top of my head, so there
    are probably errors in the details) the phone line normally sits at
    about 48V (ignoring polarity since it isn't important to this trick).
    When you pick up the receiver the phone loads the line and drops it
    down low, maybe something like 8V or whatever it is.

    The trick is for phones that you don't want to be able to pick up
    while you are using the line is to put an inline diac with the phone
    you want to restrict. Diacs are semiconductor devices (looks much like
    a diode) that when you exceed their voltage rating, they turn on and
    conduct. And they stay on until the current goes to zero where they
    stop conducting and turn off.

    So if you put the diac in line with the phone, normally you have
    PhoneLine -> Diac -> receiver switch
    When the phone is on the hook, switch is open and diac does nothing.
    Pick a diac rated around 20V or so to be in the middle of the 8V to
    48V range. When you pick up the phone, the switch is closed, the diac
    sees 48V, and starts to conduct basically connecting you to the line.
    When you hang up, switch opens, current stops, diac opens.

    Nothing very exciting there but here is where the trick comes into
    play. If the phone line is in use (say you are using your modem, fax
    machine, or talking on another phone), if you pick up the "diac'd"
    phone, because the line is already in use, the voltage on the line is
    already dragged low (say 8V). This is too low for the diac to turn on
    and therefore the phone is effectively cut off from accessing the line
    (to easedrop or corrupt/kill your modem connection). Pretty cool huh?

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