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Thread: Changing cold cathode backlighting to LEDs (+auto brightness control)

  1. #1
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    Changing cold cathode backlighting to LEDs (+auto brightness control)

    Hi everyone,

    I thought to share my experience in case someone is planning similar project (don't know if anyone has done this here before).

    So in short:
    I wanted to get dimming for my 8" 4:3 LCD backlight and easiest way to do it was to change the CCF lights to LEDs.

    What I did was ordered two PCBs for 51 SMD LEDs (1500mcd @ 25mA) and soldered two new light bars. LEDs are very close to each other so the light source is uniform and lights the LCD evenly. I'm driving the LEDs with half of the nominal current (~12,5mA) and yet it produces brighter screen than with the original lighthing, plus it consumes ~150mA less than the CCFs.
    Only drawback is that the color tone is bit greenish as the LEDs color is quite cold (>6000K).

    LEDs are driven with PWM, produced with AtTiny15L micro controller. The PWM is controlled with a LDR resistor (resistor that reacts to light) so that screen brightness is controlled automatically. There's lots of filtering and smoothing (software and hardware) so that screen wont change its brightness too fast. I still have to do some software tweaking to get rid of some annoying flickering at the dark end of the adjustment, but in general it works quite well.

    Here's some pictures of the project:


    Dimensions for LED PCB, depth is 5mm.


    One bar ready. Soldering was done with a traditional soldering iron (-station), which was a bit challenging (for me).


    LED resistors in the back of the PCB.


    LEDs @ 12,5mA vs. CCF


    LEDs mounted. Had to dremel the casing a bit as it was a tad too tight.


    Nice and even lighting. Even shows reds eventhough LEDs were really cold white toned.

  2. #2
    Constant Bitrate simplex's Avatar
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    Good job!
    I'm trying to do the same thing (check my worklog),
    and I know someone who also did this in the hungarian carpc community...
    (here's the link, sorry, it's hungarian, but the pictures could help)
    Check my worklog:
    Corsa + Atom + Gentoo Linux + 9" capacitive touchscreen

    Lord of the boards: DFI CP100-NRM

    "Or you can try Ubuntu, but than don't tell everyone you are using linux,
    because it's just a secret unreleased prebeta of Windows 3829" :P

  3. #3
    What can I say? I like serial. Curiosity's Avatar
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    Nice work. Are those all in single parallel? Looks good anyway. I did that last one that thanh510 did and love it with just those few LEDs. It's actually sunlight sorta readable. Your's must be blinding at full power. LOL With the PWM, just limit your low end to a good night time brightness. It doesn't need to get down far enough to flicker.

  4. #4
    FLAC
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    I've been trying to do something like this after thanh510, but never got around to it due to the led's size on the premade strips.
    Could you please give me more detail on your work? where did u get the PCB board? LED? and Resistors?
    Any chance you'll be sharing the controller's design? If i have to i'll settle for a simple premade controller.
    How's the sunlight readabilty?

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    Curiosity,
    LEDs are in series of three, so one 270ohm resistor for three LEDs and 17 of these on one PCB and yes, screen is quite bright at full PWM . By flickering I meant just a bug in the micro controller software as AtTiny15 has only 8-bit PWM (255 steps) and how human eye sees the brightness causes the steps to look like quite huge at low end of PWM and controller changes the brightness constantly between few final steps which is annoying when driving in the dark.

    There's lots of smoothing done with capacitors and also by software so that it changes from 100% to 0% in 10 seconds, but I also need to make another timer so that if the change is only one step, it waits for 10s or so between the changes. My assembly skills have rusted and a friend made me this software I use atm. but I'll try to get the changes done asap.

    Punky,
    LEDs are PLCC-2 SMD LEDs, don't know the exact brand sorry (I can check at home if they have any markings). Ordered them from a finnish guy who also made me those PCBs. Of course he can build stuff for you too, but I think you can get PCBs made somewhere else, closer to you. Edit: email for this guy is myynti(at)electroforge.fi
    LEDs' dimensions are ~ 3,2mm x 2,8 mm x 2mm (LxWxH).

    I can share the AtTiny15L program code here in this thread when I get it working properly. Micro controller drives SI2306 FET (SMD) that feeds the leds.

    Principle schematic is like this:
    (30k resistor can be ditched, 10ohm is too small and I replaced it with a ~300ohm one, 50k resistor is actually now ~500k and this schema also lacks all capacitors)


    EDIT: Oh, and you can make the PWM also with 555 timer circuits but I preferred micro controllers because it allows all that software filtering and tweaking.

  6. #6
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    simplex, did that hungarian guy connect all those leds with wires directly? That's madness

  7. #7
    Constant Bitrate simplex's Avatar
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    Quote Originally Posted by sniiki View Post
    simplex, did that hungarian guy connect all those leds with wires directly? That's madness
    No, THIS IS SPARTA!

    So, he used SMD LEDs, voltage is 3.6V.
    He used 3 in series, call it a line (3,6x3 = 10,8V)
    Then wired the lines parallel, and added a resistor.

    That's not the best way because if one line is out, it will kill the others slowly too, but It's easier.
    The other way, you can add an SMD resistor to every line, it's more difficult, needs more space, but more safe.
    Check my worklog:
    Corsa + Atom + Gentoo Linux + 9" capacitive touchscreen

    Lord of the boards: DFI CP100-NRM

    "Or you can try Ubuntu, but than don't tell everyone you are using linux,
    because it's just a secret unreleased prebeta of Windows 3829" :P

  8. #8
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    Quote Originally Posted by simplex View Post
    No, THIS IS SPARTA!

    So, he used SMD LEDs, voltage is 3.6V.
    He used 3 in series, call it a line (3,6x3 = 10,8V)
    Then wired the lines parallel, and added a resistor.

    That's not the best way because if one line is out, it will kill the others slowly too, but It's easier.
    The other way, you can add an SMD resistor to every line, it's more difficult, needs more space, but more safe.
    Yea, I have own resistor for each 3 LED chain so if one chain burns it doesn't affect to the rest.

  9. #9
    FLAC
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    A couple more question:
    How is the sunlight readability?
    How much is this costing you?

  10. #10
    FLAC
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    Im also interested in how much brighter than stock is it with the new LED backlights. Also, what is the benefit to using PWM to drive the LEDs instead of just running them straight off a power source, other than the ability to dim them? It seems to increase the costs and complexity for this project.

    What I was thinking was to use two super bright banks of LEDs, then have one of these banks turn on and off based on how much ambient light there is (photosensor).

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