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Cupboard alarm

  1. May 13, 2011 #1
    Prob. a bit basic for on here but this type of electronics isn't my best?


    Here is an alarm circuit I have designed to be incorporated in a metal cabinet, can't use a reed switch as the cabinet is steel.

    Quick talk through

    Op amp and the 2 pot. dividers compares the light levels inside and outside the circuit, when the one is close to the one on the right it will activate the op amp output, this will need to be set up in situ using variable resistors

    When this occurs it will activate the thyristor which acts like a latching switch, this allows current to flow to the 555 timer which will let some leds and a buzzer to work in pulses.

    This can be reset by closing the door to darken the inside of the cabinet and then turning the circuit on again

    Would this work?
  2. jcsd
  3. May 14, 2011 #2


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    Science Advisor

    A reed switch usually has to have a magnet within a centimeter of it to switch. So, the steel cabinet should not be a problem.

    Your circuit may work. Remove the capacitor from pin 5 of the 555 though, and use a much smaller one on pins 2, 6. Make the resistors 1 K and 10 K and the capacitor 0.1 uF, for a start.

    It should oscillate at about 18 Hz with the components you have given and about 680 Hz with the ones I suggested.

    I'm not sure about the light level comparisons. What if someone turns off the lights before opening the cupboard? Maybe you need a padlock.
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