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DIY LED array? Help a newbie.

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by Shang, Dec 18, 2002.

  1. Shang

    Shang New Member

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    To start off, my electronics experience is fairly limited, although I do know the very basics of electronics. I'm looking for a nice learning project for me to work on over winter break.

    Basically, what I want to do is rig up a small LED array to provide lighting for the writing surface of my esk, since I don't have room for a full-size lamp, and I hate those large clamp-on lamps. Yes, I realize that there are ready-made commercial solutions that would work, but this project is also supposed to keep me occupied for a month until I head back off to college.

    At the moment, I'm considering either a rectangular array that I can attach using a small clamp, or a long array, maybe 2' in length, but not more than 1/2" in width. I'd like it illuminate about as well as a 40W bulb, but a smaller illumination angle is fine.

    The questions I have are regarding the powering of the array, the choice of LEDs for best color results, and dimming the array.

    I briefly toyed with the idea of running the array off batteries, so that it would be portable, but a I understand, the voltage droop as batteries discharge would make the light level very inconsistent unless I rigged up circuit to maintain voltage. Is my understanding correct? Is such a circuit difficult to construct?

    If batteries turn out to be infeasible for my application, would powering the array from a wall wart be feasible if I can find an appropriate voltage wall-wart? Does current from DC wall-warts still have a frequency of 60Hz? If so, am I correct in assuming that an appropriate capacitor would smooth out the resultant flickering in the LEDs?

    Also, it's not necessary, but I would like to be able to dim the array. This would require something more complex than a variable resistor, since LEDs are "current-dependent" correct? (Is that the proper term for it? Basically, the idea that they only operate under a small voltage range.) Would something like a PWM controller work for this?

    Finally, my only concrete requirement, the array must have decent color, something in the incanescent/halogen range. I absolutely cannot stand the bluish tint that white LEDs have. Would throwing in some other color LEDs into the mix even out the color? Like a few yellow or red LEDs in addition to white ones? I also stumbled across http://ledmuseum.home.att.net/whitecap.htm. Has anybody had experience with these? I'm considering them since cheap blue LEDs are cheaper to find than white ones, although finding the filter caps may be very difficult.

    Sorry to ask so many questions, but I must've searched for information on LED lamps for 2 hours last night, without any sites that answered all y questions. Any help is appreciated. Thanks in advance.
     
  2. kinjalgp

    kinjalgp Active Member

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    Hi,
    White LEDs are also available with pure white light without bluish tint. Have a look at the LED torch at www.netinc.com. To control the intensity of light you can use PWM. A very simple op-amp based circuit can be found at this page: http://www.solorb.com/elect/solarcirc. Instead of the Motor load connect your LED array to it.
    Line frequency (60Hz) will not have visible flickering effect because anything happening in less than (1/10)th of a second is not noticed by human eye and here in this case LED will turn on and off 60 times a second. But if you are going to use PWM line voltage will be of no use. You'll require to use Transformer and rectifier to power the circuit.
    Line frequency cannot be directly smoothened out using filter capacitor because capacitor will short the AC line.
     
  3. bogdanfirst

    bogdanfirst New Member

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    :D
    i dont think i understand right....
    u could use 3mm white leds just like kinjalgp said.....high brightness...50mA or 30mA ones....
    but i dont think that u can get as much light as a 40Wlamp.....
    some good results can be achieved also if u put together blue leds, red and green and white light will form, but u need some control on each colour of leds so u can obtain white light.... :p :)
    lol
     
  4. dave

    Dave New Member

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  5. herbymcduff

    herbymcduff New Member

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    Sure if you use red, blue and green to make white light. But you have to know how much of each to use to make white light. You can't just use the three LED's to do it.
    Also, to get an idea on how that works would be to look at a projection screen tv. They seperate each color.
    And to do what bogdanfirst said, you would need something (like a magnifier) to consentrate all the colors into one, stratigically placed.
    You also probably remember when blue LED's were expensive, because they were rare. It is just the same for the white one's now, since they haven't been out that long.
    I have a head lamp with magnifiers. It works perfect for what ever you are doing. Now it doesn't eluminate the whole bench, but you should have adequete amout of light from above. The head one's range anywhere from $20 and above, just depending on the model. Not only do you get light, but also magnifiers (good for checking loose solder joints)
     
  6. kinjalgp

    kinjalgp Active Member

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    White light can be obtained by mixture of R+G+B in some proportion. But if you use point sources like LED, the chances of light being mixed up are very less and you'll get un-even spots of various colours and not evenly spreaded white light. :(
     
  7. herbymcduff

    herbymcduff New Member

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    I agree with kinjalgp on that one. LED's don't emit as much light as regular tubes, thus you would probably never really achive white light with those LED's.
     
  8. LApprenti Sorcier

    LApprenti Sorcier New Member

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    Right, and that would be rather ironic considering that the initial problem was to try and even out the blueish shades of white LEDs...

    If you still want to do it yourself, why donĀ“t you build your own lamp with fluorescent tubes, maybe even those circle-shaped... The problem of voltages and all that will still be challenging.
     
  9. bogdanfirst

    bogdanfirst New Member

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    well, i just soggested that u could use rgb and obtain white light but i didnt thing, and u r right, u will obtain a surface with white, red, blue, green spots.....
    i would go for 3mm white leds..i bought a few, there is no bluish light, there is pure white light, and some emit more light than 5mm ones.....
     

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