Focusing an LED beam

Discussion in 'General Electronics Chat' started by Shagas, Aug 12, 2013.

  1. Shagas

    Thread Starter Active Member

    May 13, 2013
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    Greetings again.

    Due to the non-availability of Low power IR laser diodes where I live , I am considering of using a focused IR Led source for my alarm system.

    I tried google but I couldnt find any good info on it.

    Basically what I would like to do is focus the light from 1-4 IR LED's into a spot about 1-3 cm in diameter at a distance of about 8-10 meters.

    Any suggestions on what kinds of lenses one should use to achieve that?


    Thanks in advance
     
  2. Shagas

    Thread Starter Active Member

    May 13, 2013
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    Do I need a concave lens with an 8 meter focal length?
     
  3. Shagas

    Thread Starter Active Member

    May 13, 2013
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    I just got another idea...

    A red laser will have some of it's spectrum in the IR range . Could I use a coloured lense to filter out some of the visible red part of the spectrum?
     
  4. MrChips

    Moderator

    Oct 2, 2009
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    I would suggest using a single IR emitter if you want to try focusing.

    You have your physics of optics wrong. For focusing you need a convex lens.
    A focal length of 8m is absurd and rare.
    What you need are two convex lenses of short focal lengths. I am guessing 3 to 5 cm.
    Simple magnifying lenses or lupes should work.

    Why two lenses? One at the emitter and the other at the detector.
     
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  5. Shagas

    Thread Starter Active Member

    May 13, 2013
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    Yes , convex . In my mind I had the convex picture , just got the name wrong.
    yeah I thought a focal length of 8m would be a no-no .

    Also I'm going to go down to the shop to get some more powerfull IR LEDs in 100-500 mw range for more power

    I don't get it , if I put a lense with a focal length of a few centimeters at the emitter , then doesn't that mean that the rays will converge at a few centimeters and then spread out to oblivion?
    What would be the point of that?

    So the lense at the reciever would simply be to concentrate the beams into a point , similar to starting a fire using sunlight and a magnifying glass right?
     
  6. MrChips

    Moderator

    Oct 2, 2009
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    When you place the emitter at the focal point of the lens the light will be transmitted in a parallel beam the same width as the lens with some divergence.

    At the receiving end, the lens will collect the beam and concentrate it on the detector, yes like focusing sunlight to start a fire with a magnifying glass. Believe me, you aren't gonna start a fire with you IR emitter.

    If you want to test your alignment, use a super bright blue LED and test this in the dark.
    In fact, you can use blue or white LED in your final design.
     
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  7. Shagas

    Thread Starter Active Member

    May 13, 2013
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    Ah I think I see , so basically i'm spreading an uncoherent beam into a parallel beam which has a width of the lens and then i'm concentrating it back into a point right?

    Heh , i'm not afraid to start a fire , I was just making an analogy :)

    Shouldn't I use a bright RED led to test my alignment (since i'm going to be using IR , it's going to be the closest) ? If i'm not mistaken , light wavelength affects the spread angles etc ? (yes , my knowledge of optics is craptastic)

    By the way this is for a Laser alarm system that I have posted here:
    http://forum.allaboutcircuits.com/showthread.php?t=88075

    So i'm going to have to keep it in the Infra red.
     
  8. MrChips

    Moderator

    Oct 2, 2009
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    The reason for using IR instead of visible light is so that a would-be burglar does not notice the alarm.
    Also it is less sensitive to ambient light.

    You can ignore the effects of wavelength otherwise.

    Of course, you will match the detector to the emitter with respect to wavelength.
     
  9. Shagas

    Thread Starter Active Member

    May 13, 2013
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    Also because I'm using a 38khz IR detector module as the reciever

    Thanks for the help and info Mr.Chips!
     
  10. MrChips

    Moderator

    Oct 2, 2009
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    Sorry, you are correct about the effects of different wavelengths.
    I forgot that the refractive index is very dependent on wavelength.
    Yes, use a red LED for testing.
     
  11. Shagas

    Thread Starter Active Member

    May 13, 2013
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    Yes , refractive index is the term I was looking for . Thanks for the confirmation!
     
  12. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    Are you reinventing the wheel?

    What you want seems to be the standard photointerruptor system like those used for safety stopping a garage door. In fact you can pick up one of those exact systems on ebay for $10 to $30.

    You can probably get away with the emitter detector tandem package. Not as great for safety because the other side is a refractive reflector, type of reflector that is on a kids bike. Physical interruption with a reflector might not cause a circuit interruption.

    If it is good enough that setup will save you a some wiring hassle because the emitter and detector are in one harness. Especially if you intended to to poke holes in walls and fish your wires through, that would be a time saver.
     
  13. mattbullet

    Member

    Nov 29, 2008
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    Hi ;
    About 30 years ago now, I had picked up an ir detector card from
    Radio Shack . I think it had zinc sulfide in its screen. You would charge
    It with white light then use it to test a remote or other ir source.
    It would glow a kind of orangy-red.
    Something like this would help you "see" your focusing attempt.
    I'm certain these are still available.

    Matt
     
  14. LDC3

    Active Member

    Apr 27, 2013
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    No, that would not work. A laser usually emits light of one wavelength only.
     
  15. Shagas

    Thread Starter Active Member

    May 13, 2013
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    Thanks for the info , but I prefer to make the thing myself and learn alot from it (which I already did ) . The only component missing from the picture is that 950 nM laser diode .I've got everything else ready and programmed. The only reason i'm jumping through ridiculous hoops here is because I'm trying to find a way around the 3 weeks that it would take for that diode to arrive from china .
    Also learning in the process of doing that. Buying something like that would take the fun out of it
     
  16. Shagas

    Thread Starter Active Member

    May 13, 2013
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    Nice , i've heard about those in the past but you can also see Ir with a phone camera although not so clearly.

    Anyway , I got some more powerfull Ir LED's which i'll test out tomorrow . But so far it doesn't look promising so I think that I'll be scrapping that and just waiting for that IR LD
     
  17. Shagas

    Thread Starter Active Member

    May 13, 2013
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    yeah , you are probably right , I was hoping these cheaper red ones would be abit more dirty but I guess they can't be +- 300 nm dirty
     
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