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Break beam technogly

Discussion in 'General Electronics Discussion' started by lapistola, Dec 27, 2011.

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  1. lapistola


    Nov 22, 2011

    I have a project in mind that requires sensing a pellet fired from an air rifle while in the air. Basically its for a target system that auto scores you.

    My first thought was to use infrared leds and sensors but as I need to create a grid of beams with no more then 4mm spacing I think IR will not work. So im onto an idea with lasers and photodiodes.

    I plan to use an MC that loops very little code then uses functions to workout the score once 1 or more beams have been broken.

    My questions are:

    1, is there a better break beam method more suited?
    2, if not is there a laser that will shoot out a flat beam say 6-8mm wide?
    3, will the photodiode react fast enough as the pellet will only break the beam for a nano

    Naturally using a switch that requires a pellet to touch is out the question. These high power rifles have gone through metal thats designed to catch the pellets.
  2. eptheta


    Dec 20, 2009
    I don't have the expertise to tell you if this will work, but if you have the time to experiment, this is what I'd try.
    This may not work if the target size is large..
    -2 linear arrays of LEDs (of two different wavelengths perhaps) on adjacent corners of your target.
    These have to be physically bound on the sides so that it's wavefront more or less represents parallel planes of frustums of cones.
    -On the other side, arrays of photodiodes or LDRs that have been calibrated using their response graphs to generate an output for the wavelength opposite to it.
    -Your pellet should be able to break this grid and generate an output.

    --Of course, your grid width depends entirely on your physical spacing of the receiving LDRs (since the light beam will be more or less uniform along its length)
    --You could use prisms to diffuse the light uniformly along the width of the target by compromising on intensity.
    --You might as well use IR if you are confident enough that your wavefronts will only reach the opposite side.
  3. lapistola


    Nov 22, 2011
    Thanks. My fall back was using LEDS in an array across the top and one side. From what I understand LDD's are faster then LDR's which is my meaning for use. The light source will only be 5" away from its sensor.

    Im not confident that IR has a fine enough beam nor there receivers being able to react fast enough.

    It would also be great if I could set up a second circuit that beam was broken to work out how fast each gun is firing.

    I do need to test first but wanted to know which option to order components for.

    IR isn't expensive so maybe ordering a few options may be an idea.
  4. davelectronic


    Dec 13, 2010
    Why not look into the workings of a chronograph maybe you could adapt a circuit based on that principle.

    If your Rifle penetrates a pellet traps steel case your gun must be more than 12 ft/lb energy, more like 18 - 20 + ft/lb, i noticed your in the UK you need an FAC if not you could end up in shtook . An air weapon of greater than 12 ft/lb requires a firearms certificate as the rife is seen in the UK as a high powered firearm. Only making you aware of this. :)
  5. Laplace


    Apr 4, 2010
    On question #3: The upper limit on pellet velocity is about 1000 ft/sec, whereas the thickness of the light beam is probably around 1 mm. Velocity = 1000 x 12 x 25.4 mm/sec, so the time it takes the pellet to move across the beam is 1/(304,800) = 3.2 microseconds. Any PIN photodiode should respond more quickly than that.

    I once built a ballistic chronograph where the optical sensor was a linear array of ten 0.5 mm plastic optic fibers and the other end of the fibers were collected into a tubular ferrule and using a tiny glass lens to focus the light onto a PIN photodiode. The pellet would interrupt the light going to at least one of the fibers (1 of 10) and the resultant 10% drop in light intensity was amplified and then used to start or stop the timer. Using the plastic fiber was easier, though somewhat less accurate, than using an optical slit.
  6. lapistola


    Nov 22, 2011
    Dave them pellet traps are not thick and when your shouting a .22 domed head pellet at them from 11 meters, after a 100 shots its weakened the metal enough to blow though them, I should only fire flat heads at them as they don't do anywhere as much damage. All our guns are legal and brought new from a local shop thats well established and an official supplier. All our guns are registered and under the UK legal limit.

    As an example of the pellet I use to the pellet I should fire at traps, when a domed head is fired at a tin can it goes through the can without moving it, when a flat head fired from the same gun at the same distance hits the same can it goes through the can but also throws the can up and over. Basically its my fault but prefer that type of pellet as it seems more accurate from my preferred riffle. I was aware of the UK limit and the requirements, but thanks anyway I know you mean well!!

    Thanks both ill look into a chronograph!
  7. davelectronic


    Dec 13, 2010
    Yes after that many shots at close range i have seen the traps torn through.
    Wish i was still shooting, use to do air rifle and .22 Rim fire competition firearms .Good luck with your project, happy shooting. :)
  8. lapistola


    Nov 22, 2011
    Its good fun Dave, Do you not have room at home for a pistol 5m range with a .177?
    I like playing with pistols but my main enjoyment is long range scoped riffles.

    Thanks for the Chronograph tip, perfect for the speed and will incorp. I just need to figure out how to do the auto scoring without spending stupid amounts or having loads of beams.
  9. davelectronic


    Dec 13, 2010
    Unfortunately not ive got a typical 3 bedroom terraced house, small yard etc houses backing on to the rear of my place, you know how quick they are to react over here in the UK, no sooner i go out in the yard with an air pistol and a good back stop, next thing you know i will be looking down the business end of an MP5.

    I use to shoot three days a week, an outdoor range in our local park king George 6th park in Ramsgate Kent, and the indoor range at ST. Lawrence college in Ramsgate, the college withdrew there range access after a pupil was nearly killed in an RTA by a ambitious member driving like a mad man, so that went, the parks range was revoked after the terrible gun crime over here as i am sure you know about, the council took that away.

    So no shooting for me, any way happy new year, all the best with your project, and have fun shooting. :)

    PS. Because the range in the park was match Rifle .22 Rim Fire, expect you have heard of Bisley its got history, many years.
    Last edited: Jan 1, 2012
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