LED Backlit lightbox (first project)

Discussion in 'The Projects Forum' started by GarthFader, Jun 16, 2010.

  1. GarthFader

    Thread Starter New Member

    Jun 16, 2010
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    Hey everyone,

    As the thread title suggests, I'm looking at building a pretty basic unit as my first project. A simple lightbox with a LED panel as a light source, run from the smallest/lightest DC power source I can fine, like a couple of 9v batteries.

    The box isn't large, 300mm x 200mm (approx 12" x 8").

    I was hoping to get away with 35 x 5mm LED's for the lighting, which would be behind a diffuser panel of 1mm opal plastic.

    The advice I am looking for from you, the gurus is as follows.

    1. What intensity would my LED's need to provide sufficient backlighting?
    2. What would be the best way to configure them to get a good balance between burn time and brightness?
    3. What type of power source do you recommend?

    I appreciate any help you can offer, as I am quite a noob.

    Cheers!
     
  2. Markd77

    Senior Member

    Sep 7, 2009
    2,803
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    I think finding the brightness (current) required would be trial and error because we don't know your requirements (just use 1 LED behind your diffuser). White LEDs have a tendancy to go funny colours when driven at low current so PWM might be required.
    9V batteries are pretty bad. Lithium ion batteries are very light for rechargables but hard to charge. Lithium AAs are very light and high capacity if you want disposables, but expensive. NiMh AAs are heavier and lower capacity but might be an option.
     
  3. GarthFader

    Thread Starter New Member

    Jun 16, 2010
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    Thanks for the reply, I see your point regarding the brightness, I'll just have to try a few til I find the result i'm after.

    Once I do i'll check back in with the specs of the LEDs for some further advice.

    And excuse my ignorance, but what do you mean by "PWM might be required"?
     
  4. Markd77

    Senior Member

    Sep 7, 2009
    2,803
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    It's just flashing them very fast with a variable on time so they appear dimmer (and use less power). There are circuits with 555 timer ICs in the reference at the top of the page that will do it.
    It might not be required, but if you find the colour of the white LED is unsatisfactory at low current, it is a way to improve it.
     
  5. Bychon

    Member

    Mar 12, 2010
    469
    41
    Trying to be a bit simpler than Markd...PWM is a fast switching circuit such that the LED's get full voltage, and thus full brightness and proper coloration, for a part of each second. The switching is way faster than humans can detect, so it looks like nothing is happening except the colors are right...and you might not need it.

    ps, when you get settled on which LED's you want, you can check back here about how to wire them up. Series, parallel, all that stuff.
     
  6. GarthFader

    Thread Starter New Member

    Jun 16, 2010
    7
    0
    Awesome.

    The intensity of the lights isn't critical, but yeah, I will need enough to do the job.

    The project is to create some street art pieces with a tech edge. I will be installing them at odd places with some of my artworks in the lightbox. Eventually the batteries will die and the unit will fall apart and become urban waste... but I'm happy with that.

    The next revision is to include a small solar cell to recharge the battery and a photodiode to only switch the light on when it gets dark, so the unit could run forever, or at least until something fails.
     
  7. n1ist

    Active Member

    Mar 8, 2009
    171
    16
    You also will need to play a bit with the distance between the LEDs and the diffuser, and the LED's viewing angle. Too close or too narrow, and you get a very uneven pattern with hotspots over each LED. Too far, and the brightness suffers.

    As for street art, just make sure you don't cause another Mooninite scare :)
    /mike
     
    Last edited: Jun 16, 2010
  8. GarthFader

    Thread Starter New Member

    Jun 16, 2010
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    Wow, I hadn't heard of that incident before now, and honestly hadn't though of the implications placing something like this in public might have.

    Thanks for the heads up.
     
  9. GarthFader

    Thread Starter New Member

    Jun 16, 2010
    7
    0
    Alright, I have the first piece of the puzzle... the LEDs.

    I will be arranging 35 of them in the array. The specs are as follows.

    Size: 5 mm
    Lens Color: Water Clear
    DC Forward Voltage: 3V
    DC Forward Current: 20mA
    Color Temperature: 6000-6500K
    Luminous Intensity: 15,000MCD
    Viewing Angle: 15 ~ 30 degree
    Lead Soldering Temperature: 260°C (≤5 s)

    I want to look at 2 options for running them, one being a couple of 9V batteries ganged up, and the other a DC transformer running off a 240v (Australian standard) wall outlet.

    Any advice?
     
  10. sceadwian

    New Member

    Jun 1, 2009
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    Sorry bychon, I haven't met a PWM unit that I can't visually see under the right conditions. Most people can't call out that they can see flicker, but trust me, it bugs the crap out of them.
     
  11. coldpenguin

    Active Member

    Apr 18, 2010
    165
    9
    You are saying that you can see something that is only lit for 6us?

    Are you confusing POV displays for PWM?
     
  12. GarthFader

    Thread Starter New Member

    Jun 16, 2010
    7
    0
    So... can anyone offer assistance regarding the setup of these?

    I have them laid out in 5 rows, each row with 7 LEDs.

    I was thinking of running each row in parallel and then the rows in series, is this a practical way to set them up? My thinking behind this is that is one dies, only the row containing that LED will drop out.

    What are the advantages and disadvantages of setting them all up in parallel or all up in series.
     
  13. GarthFader

    Thread Starter New Member

    Jun 16, 2010
    7
    0
    I have rummaged around at home and found a 12v transformer, with output values 12v, 500mA, 6VA.

    Would this be suitable to use as a power source?
     
  14. Wendy

    Moderator

    Mar 24, 2008
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  15. sceadwian

    New Member

    Jun 1, 2009
    499
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    Not sure cold penguin never tried 150+khz before. Considering the modulation depth on an LED is full on vs full off, it were moved through a persons field of vision and the off time was high enough a person should be able to notice that it's not a steady light.
     
  16. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    Garth, you are most definitely going to want to go with the AC adapter(Wallwart). If you are running 5 strings of 7 leds, you will want a wallwart that can handle:

    7leds @ 3v = 21v+3v (for good luck) = 24vDC
    7leds @ 20ma = 140ma (min)

    So a 24vDC 500ma will work great for you.
    The amperage can be higher, but I wouldnt go lower.

    Here is one for a pretty good price:
    http://www.ioffer.com/i/24v-2-65a-65w-apple-ibook-ac-adapter-w-au-power-cord-114856811

    I would also ask around, old printer power packs and such run on 24vDC wall warts.
     
  17. n1ist

    Active Member

    Mar 8, 2009
    171
    16
    For 5 strings of 7 LEDs, the current is 5*20mA = 100mA.
    /mike
     
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