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building a window alarm

Discussion in 'Electronic Basics' started by biner, May 20, 2004.

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  1. biner

    biner Guest

    Hello All,

    I need to build an alarm that would make a noise (not necessarly
    big, just enough to wake me up) if somebody went through a window in
    another room. So I would need a detector on the window that is linked
    by remote to a receiver in my bedroom. For the detector, I was
    thinking about a light path but with the wind and curtains it may not
    be a good idea. I think the best thing would be a device that would
    detect if a small wire is broken.

    I would like to use this project as an opportunity to get back into
    playing with electronics, something I have not do since quite a while.

    Have I underestimated the amount of work requested. Does anybody
    have a pointer on what (and where) to check in order to build that
    thing (website? Plans of circuit). Any pointer would be nice.

    Thanks in advance.

    btw if I am in the wrong group for this question please tell me where
    I should put it. Thanks.
  2. grahamk

    grahamk Guest

    Have a look at

    S2 could be a switch, a piece of wire, or a magnetically operated switch etc.
    B1 could be in your bedroom.
  3. The detector (sensor) could be of various kinds, and you could have
    your alarm operated if any of them was activated, or if a combination
    of several of them were simultaneously activated. Possible sensors
    could be:

    - microswitch operated by window opening

    - magnetic switch (reed switch in window surround activated when
    flush-mounted magnet in window is moved)

    - sound switch, filtered, in anticipation of window being broken

    - interrupted light beam, using UV for extra reliability; with care,
    the false alarms you're apprehensive about could probably be avoided.

    - pressure mat under window

    - proximity detector

    As for communicating that activated sensor signal to your bedroom, why
    are you apparently thinking about wireless or remote control? What's
    wron with a pair of wires? OK, so you may have to drill a hole, or
    route it under the carpet and out into the hall etc, but it is
    reliable, simple and inexpensive.

    You'll find hundreds of potentially relevant circuits and ideas using
    'burglar alarm circuit' in a google search.

    When you've narrowed it down a bit, come back for more specific help.
  4. And I meant to add motion detector (a familiar PIR unit, hacked for
    the purpose.)

    By 'proximity detector' I meant a capacitive sensor. Like the one I
    designed for several touch lamps, shown here.

    It's easily adaptable for burglar alarm purposes, but for reliable
    operation requires human contact within about 2mm of the sensor. (So
    kitchen foil underneath a insulating surface is one possibility.)

    Of course, a key factor determining your approach is your skill level.
    Even a relatively simple alarm can be demanding if it's to be
    reliable. Have you thought about buying a ready made kit?
  5. JeffM

    JeffM Guest

    Possible sensors:
    I wondered where PIR was on your 1st post. :cool:

    Under-carpet mat is the simplest (a supervised loop is necessary).
    PIR (Passive InfraRed) is the most reliable.
    Having to replace a broken trip-wire at 3AM because of a false alarm
    would quickly reveal a bad design choice.

    Well worth consideration.
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