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simple alarm system ideas?

Discussion in 'Electronic Basics' started by Spork, Sep 1, 2004.

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  1. Spork

    Spork Guest

    hello. I would like to install a very simple alarm system. I've been reading
    a little about the professional models online and they have to many features
    and most have very poor documentation for a beginner.

    I was thinking of something simple like magnetic contacts on all exterior
    doors wired to a piezo buzzer by my headboard.

    I'm assuming I would use normally open contacts wired in parralell then
    wired in series with a piezo buzzer? Do I need resistors or anything else?

    I realise the alarm would cease if the wires were cut or the door was shut
    but it would be enough to wake me up and hopefully startle the thief.

    My other question is can those radio shack surface mount contacts be used on
    a steel door with foam core without using some kind of stand off? I've heard
    you can and cant so I dont know what I should do.

    I appreciate any other ideas. im just learning about electronics so I want
    to keep it simple.

  2. I also would be interested on how to link this system up to a computer for
    logging etc of movement.

  3. peterken

    peterken Guest

    Use normally closed contacts, and invert the signal using a relay or
    transistor for controlling the buzzer.
    That way, cutting the wires sounds the alarm.

    If using a relay, connect all switches in series, and one end to +line and
    the other to the relay coil.
    Connect the other end of the relay coil to -line.
    Connect the buzzer to the +line, and the other end to the NCcontact of the
    Connect the common of the relay to the -line.

    If using the transistor, use a series resistor of say 2k2 in the base
    connection (6-24V circuit).
    Transistor can be something like a BC558 or BC337, a simple NPN-type of
    minimum say 100-200mA.
    Connect the buzzer to the +line and collector.
    Connect the emitter to the -line.
    Connect all switches in series, and one end to +line and the other to the
    base-resistor of the transistor.

    For as well transistor as relay circuit, the buzzer voltage should be as the
    power supply.

    Linking the system to a PC is fairly easy, using a simple off-the-shelf
    multi-IO board.
    That way, using a little programming, all circuits can be controlled
    Most of these boards have a simple program delivered, so it's as easy as
    connecting and running the program.


    logging etc of movement.

  4. Thanks - so would a PIC chip is what I would need?
  5. Tom Biasi

    Tom Biasi Guest

    Wire the contacts in series with a relay coil and the relay's own contacts
    so that when the relay drops out it can not be reset until you reset it.
    You would need a relay with two contacts. SPDT (single pole double throw)
    The coil voltage should match your supply voltage. The supply should first
    go to the relay's common. The N.O. (normally open, the one open when the
    relay is not energized) goes to the relay coil. The N.C. contact goes to the
    sounding device.
    The trip contacts are in series with the relay coil. You have to manually
    arm the alarm. Put a N.O. push button switch between the relay common and

  6. Spork

    Spork Guest

    would it be safe to use a 12 volt wall wart for this project? I was thinking
    I could use a louder piezo and not worry about batteries. would that be

  7. Rich Grise

    Rich Grise Guest

    Sure, as long as you don't stick your tongue between the contacts. ;-)

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