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Tripwire "mine" for CCF

Discussion in 'General Electronics Discussion' started by Birdy, Jun 21, 2011.

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  1. Birdy


    Jun 21, 2011

    This is the first time I have ever tried to design something, and I am a bit stuck. I am a member of the CCF section in my school, and a fun exercise is to make cadets go along a path with tripwire attached to "bombs". We usually use Party poppers or rape alarms for this, but neither work very well: party poppers are too unsensative, and won't go off, or if its raining, dont work. They also take a long time to reset. Rape alarms are expensive and stop working after very few uses.

    So I'm sure you've realised what I want to build by now then. Here is what I want:

    - 1st and formost, cheap.
    - I want it to be sensative
    - I want it to have a buzzer / alarm loud enough to give whoever sets off the tripwire a shock
    - I want the buzzer/ alarm to turn off after 2 or 3 seconds so it doesn't annoy anyone who lives next to the school and drain batteries.
    - It should work at low voltage - ideally less then 2 aa batteries. Alternatively I thought that a large capaciter could be used.
    - The tripwire must be able to seperate from the "bomb"

    I did have a few thoughts of how this could be done:

    -a magnet on the end of the tripwire, which is pulled away from a reed switch. I don't know how I would turn the reed switch from turning something on, to turning something off though.
    - a tilt sensor attached to an arm which would be pulled by the tripwire. I have no idea how to use a tilt sensor though.
    - An accelerometer. This would also be reusable. Freescale make a cheap one but I wouldn't know how to turn the digital output into the buzzer buzzing.

    Anyway, any help or thoughts would be much appreciated,

  2. BobK


    Jan 5, 2010
    That sounds like a fun project.

    For a "sensor", how about just using two springy metal contacts that would normally touch, and inserting a piece of plastic between them tied to the trip wire? I think you could make this quite sensitive.

    For an inexpensize way to get a loud sound look up "Sonalert". They come in many models, different voltages, frequencies and loudness.

    A simple 555 timer circuit could be used to turn it off after a couple of seconds.

    I think you could put something together for a few dollars per unit.

  3. Birdy


    Jun 21, 2011
    That's a beautifully simple idea.

    Dollars? You mean Pounds (but thankfully not euros!).
    Do distributes generally let you just buy a few at a time? They don't even list prices on the UK's distributor's website.

    555 timer should be easy enough.

    My main thought now is that using enough batteries to get a reasonable voltage for the buzzer would cost more then the rest of the unit. Does anyone have any thoughts on the use a capaciter?

    Thanks Bob,

  4. Birdy


    Jun 21, 2011
    I have just discovered that they sell 9v (pp3?) zinc chloride batteries in Asda for 75p, so I think that would be easier and cheaper then trying to use capacitors.
  5. poor mystic

    poor mystic

    Apr 8, 2011
    We regret the loss of 'touch powder' from the toybox.
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