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Magnetic switch connection problem.

Discussion in 'Electronic Design' started by [email protected], Jul 24, 2007.

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  1. Guest

    Hi to all.
    I have a simple(I think) problem that I'm hoping someone can
    help me with.
    I want to connect a remote transmitter / receiver pair ( garage door
    opener type thing) to an alarm system magnetic switch.When the door/
    window is opened it sends a signal to the receiver and activates the
    relay.
    Hope I'm getting the idea across.
    I only have normally open switches. If the door/window is closed and
    the magnet is near the reed switch it is CLOSED.
    Does anyone have any idea how I can connect the xmitter to this
    arrangement so that it is turned on when the switch becomes open.
    It must draw no power while closed.(battery operation)
    The obvious answer is use a switch that is open when near the magnet
    and closed when moved away(N/O) , and connect this to the button of
    the xmitter , But I don't have this type of sensor.
    My initial idea was to connect the xmitter from Batt+ to gnd through
    a PNP transistor(emitter) , collector to GND. The base connected to
    Batt+ via mag switch and the base to GND via a resistor.The switch
    keeps the pnp off until it opens and the resistor turns the transistor
    on.Problem it the constant current draw via the switch and resistor.

    Any clever ideas how to get this arrangement to work?
    Hope I'm not being to vague in my description.

    Cheers
    Rob
     
  2. John Fields

    John Fields Guest

     
  3. Guest

    Thanks for the tip John , but it still does not help me. I probably
    did not explain myself well. How can I connect the switch so that when
    it opens the xmitter gets powered. I'll short the button on the
    xmitter so that when power is applied
    it transmits.

    Cheers
    Robin
     
  4. ehsjr

    ehsjr Guest

    The real issue lies in exactly what you said:
    "The obvious answer is use a switch that is open when near the
    magnet and closed when moved away(N/O) , and connect this to
    the button of the xmitter , But I don't have this type of sensor."

    John's suggestion converts the sensor you do have into the
    sensor you don't have.

    It provides the function you want by modifying the
    switch, not the circuit.

    Ed
     
  5. Guest

    Ahh , got it now. Thanks for the help.
    Cheers
    Robin
     
  6. Jamie

    Jamie Guest

    Run a solar cell on the roof to keep batteries charged.
    Make sure it's hard to get to.
    Use a MOS Type switch which requires very little current
    to operate the gate and you then simply short the bias source
    with your switches in series, one end of the string to common and
    the other end to the input sensor..
    Look for Logic FETS, HexFets ring a bell.
    they have many. you need to look at the turn off/ON resistance etc..
     
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