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

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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
    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

    Comment


    • #3
      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.

      Comment


      • #4
        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?

        Comment


        • #5
          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.

          Comment


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

            Comment


            • #7
              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

              Comment


              • #8
                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.

                Comment


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

                  Comment


                  • #10
                    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).

                    Comment


                    • #11
                      Can you or the Finnish guy make LED bars (and maybe the controller unit)
                      for me if I tell you the size of the screen? (for cash of course)

                      I have many other things to do on the car, and maybe I can save some time this way
                      (also, I don't want to re-invent the wheel, and I'm not really into SMD technology/soldering)
                      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

                      Comment


                      • #12
                        About brightness:
                        It's hard to say any multiplier how much brighter the screen is than before as I didn't get any comparsion pictures for it because of some issues:
                        side story:
                        This chinese miracle screen takes the +12V via VGA cable from pins that should according to some standards be +5V. Foolishly I plugged this screen to my desktop PC with regular "all pins connected" VGA cable and the result was a broken HD4870 card and broken DVI input of my 24" monitor. At the time I didn't have suitable VGA cable to run this screen with PC as the original cable is attached to my car, so I never got change to feed test image to this monitor and couldn't take comparsion screens unfortunately.


                        It's also been really ****ty and cloudy weather in finland lately so no real sun light tests for now. But as the screen was quite readable in sunlight with original backlight, this can't be any worse. When comparing this screen next to my 24" samsung 2493HM, it's quite as bright and the samsung is REALLY bright. I'd bet this beats most of the CCFL screens they sell for cars (in brightness).

                        I have Canon Ixus860, Ixus40 and S5 IS cameras so if someone wants I can take a picture of the screen in full brightness with some specific exposure/aparture settings and if someone has one of those cameras, you can take a comparsion picture with same settings.

                        Costs:
                        Leds cost me 35 euros (102 of them), 2 PCBs and resistors 25 euros, but I got some discount from this nice guy from electroforge who also helped me along the project.

                        You can order the leds from any big led store and I'll bet you can get them cheaper. Cost of custom PCBs varies a lot and you should compare the prices and what they include. Few stores that you could check (took the list from a finnish forum):
                        http://ourpcb.com/
                        http://olimex.com/pcb/index.html
                        http://www.batchpcb.com/
                        http://www.goldphoenixpcb.biz/
                        http://www.pcbcart.com/
                        http://www.futurlec.com/PCBService.shtml

                        AtTiny15L micro controller cost less than 4euros, FET for power supply less than 1 euro, so the PWM really doesn't do much for total price.

                        Of course if you don't want multistep dimming it isn't any good for you and you get no benefits of using it.

                        BTW. in those led banks the leds are generally quite far apart each other so the light source won't be that uniform and you might not get that even lighting atleast in the edges. Something that you need to keep in mind.

                        Simplex, I don't have "hot air soldering iron" (don't know what's it called in english) so the result with these tiny smd components isn't that professional and it isn't that fun either So I'd politely refuse from this honor. Of course you can ask from the electroforge guy an offer how much would it cost if he makes the PCBs and puts it all together for you.

                        As for the micro controller, I'll atleast share the code here when it's done and maybe I could send a pre-programmed chip for you. Tho I cant give any promises when that would be as I'm quite busy at the work and also have stuff to do at home + I still have to finish the code so it works without glitches.

                        Comment


                        • #13
                          1. This is very interesting to me. I assumed that had to take the entire panel apart and replace everything behind the glass in order to get LED backlight. But according to you it seems like you just removed the CCF tubes and inserted LED's! Have you got pictures that shows that before I take my own screen apart?

                          2. I was under the impression that LCD's with LED from the start is sourced with 9-13V 100mA-600mA. I have not investigated further, but I know there is a device to drive LED's like that outputs that voltage and current due to controller input from 0-5V. Have a look at http://www.alpincorp.com/led_driver.html If I have understood your setup you have full brightness att 10.8V and 350mA? Then your LED's would break with a driver like that?

                          3. I do not see why to chop the voltage since you will get high power loss in the series resistors anyway.... Is this beacuse your CPU lacks DAC?

                          Ps. My interest in this is because I have designed a LED drive circuit myself, that will take dashinstrument backlight voltage as control. It can also be set by message from PC. It is configurable, and is constant current. Pictures are always nice :-) (As you can see it is not just LED drive)

                          My car installation mp3car thread "showing off project"

                          Comment


                          • #14
                            Originally posted by sniiki View Post
                            About brightness:

                            Simplex, I don't have "hot air soldering iron" (don't know what's it called in english) so the result with these tiny smd components isn't that professional and it isn't that fun either So I'd politely refuse from this honor. Of course you can ask from the electroforge guy an offer how much would it cost if he makes the PCBs and puts it all together for you.

                            As for the micro controller, I'll atleast share the code here when it's done and maybe I could send a pre-programmed chip for you. Tho I cant give any promises when that would be as I'm quite busy at the work and also have stuff to do at home + I still have to finish the code so it works without glitches.
                            Thank you anyway, I know, its madness to do all the stuff
                            I tried to persuade my father to help me with the soldering, here's a part of our conversation:
                            Me: -Hi dad, can you help me with the soldering of some SMD LEDS, and stuff?
                            Dad: -How many?
                            Me: -About 100.
                            Dad: -F**k off, son
                            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

                            Comment


                            • #15
                              I can solder if you send me the parts. I have microscope and all sort of tools for doing this. As long as it is only a few units. Is a pcb without diods a reasonable compensation? :-)
                              My car installation mp3car thread "showing off project"

                              Comment

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