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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

    Birdy

    3
    0
    Jun 21, 2011
    Hi.

    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,

    Birdy
    -
     
  2. BobK

    BobK

    7,682
    1,689
    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.

    Bob
     
  3. Birdy

    Birdy

    3
    0
    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,

    Birdy
     
  4. Birdy

    Birdy

    3
    0
    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

    1,074
    36
    Apr 8, 2011
    We regret the loss of 'touch powder' from the toybox.
     
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