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N/C Switches to turn on stuff

Discussion in 'Electronic Basics' started by Andrew Howard, Feb 25, 2004.

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  1. My work wants me to whip up an alarm system for the back door, which
    sometimes gets left open accidently.
    I was planning on hooking a power supply to a magnet switch and a siren but
    I have encountered a problem.

    The switch is a N/C one but I want the alarm to sound when the door is open.
    I originally thought of using the switch in parallel, to short the power
    supply when the door is closed, but I don't want to constantly use the power
    like this, in case I use a battery.

    Can anyone offer a simple solution to this?

    Andrew Howard
  2. Don't know how "simple" it would be, but... Would it not be a
    wiser investment to install a real intrusion alarm system? The beauty of
    a centralized system is that you can also hook in fire and smoke
    sensors, and that it can report to a central station when tripped.

    There are door-latching mechanisms available that have a built-in
    alarm which sounds any time the door is opened. You may want to look at

    Happy hunting.

    Dr. Anton T. Squeegee, Director, Dutch Surrealist Plumbing Institute.
    (Known to some as Bruce Lane, ARS KC7GR,
    kyrrin (a/t) bluefeathertech[d=o=t]calm --
    "If Salvador Dali had owned a computer, would it have been equipped
    with surreal ports?"
  3. Beau Schwabe

    Beau Schwabe Guest

    Those "fire and smoke sensors" might come in handy when he is shorting
    his power supply - grin

    Save yourself a headache and find a switch that is Normally Open!

    -Beau Schwabe
  4. Use the NC switch to operate a SPDT relay, then use the relay's NO
    contact to operate the alarm.
  5. Andrew, in order to determine if the switch is still closed, you have to
    push some current through it. Thus, with an n/c switch, you will end up
    wasting some electrons.

    However, if you waste, say, 1uA, it'll take years before your battery dies
    (it'll die of old age before you drain it with 1uA.) Thus, use a transistor
    like this:

    (view with courier font, paste into notepad, choose format:font:courier)

    VCC=9V or so
    | |
    .-. |
    10 MEG| | .-.
    | | ( X )
    '-' '-'
    | |
    +----+ |
    \ o | |
    \ | ||-+D
    \. | ||<- N-MOSFET
    o +--G||-+S
    | |
    created by Andy´s ASCII-Circuit v1.24.140803 Beta

    When the switch is closed (which is usually is) the current flows, and so
    the voltage at the gate of the mosfet is equal to the voltage at the source
    pin. Thus, the N-MOSFET is off. However, when the switch opens, the voltage
    is (slowly) pulled up to VCC, and the mosfet is turned on.

    ['Slowly' means that the voltage will come up in 10ms, which means that if
    you are sensing this voltage change using digital logic, you should use a
    schmitt trigger, cause otherwise the slow transision could cause false

    Use a mosfet that will turn on at < 5V. If you need to drive AC, use the
    current through the mosfet to drive a relay.

    This works because MOSFETs don't use current, only voltage, to turn on or
    off, so that sensing current can be really small.

    Bob Monsen
  6. Outrider 141

    Outrider 141 Guest

    Shouldn't the N/C give you an output when it opens - use of that to
    give you a ground for your ckt, essentially turning it on when the
    door opens? Could be off on that, haven't used those much.

    Use the usual techniques to reply via email.

    Molon Labe!
  7. Those "fire and smoke sensors" might come in handy when he is shorting
    I meant through resistor , or something... buggered if I knew what I was
    talking about.

    It's preferable to use the one I chose earlier, but I suppose I might be
    able to mount one of those external ones on the top of the door frame.

    Meh, whatever

    PS. The reason I have been asked to make one from scratch is because my boss
    is to cheap to buy a proper one, and there aren't any professionals that
    want to go near the place.
  8. Use the NC switch to operate a SPDT relay, then use the relay's NO
    Is it possible to do this using only one power supply?

    Oh, and thanks for everyones help so far.
    Andrew Howard
  9. Yes - just select a relay with a coil voltage the same as the alarm.
  10. Well, the solution to my problem has been found. I told my boss two options
    and he suprisingly chose the easiest one for me. He chose the way that
    allows me to use a N/O switch, probably because it cost less. :)

    hmm... Now I just have to learn how to route wires so things don't stuff up.

    Thanks for all of the help on this matter.
    Andrew Howard
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