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

  1. #11
    Constant Bitrate simplex's Avatar
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    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

  2. #12
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    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.

  3. #13
    Maximum Bitrate bes51659's Avatar
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    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)


  4. #14
    Constant Bitrate simplex's Avatar
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    Quote 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

  5. #15
    Maximum Bitrate bes51659's Avatar
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    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? :-)

  6. #16
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    simplex haha, nice and warm dad-son conversation

    bes,
    1) I tried to design the led board so that it would fit to the panel without modifications but in the end I had to dremel out a bit of the metal plate in the back of the lcd backlight diffuser and that was because I mounted the resistors on the back of the PCB. If I had mounted them in front side, it would have fit without cutting the metal.
    Here's a closer pic from the groove where the light sits. You can see the diffuser plexi glass there (~5mm thick).


    2&3) My 102 leds eat ~420mA in total with resistors and controller which is ~150mA less than the original backlight circuit. I'm fine with that. The resistor is there because the PCB maker advised me to half the current (as LEDs are driven by current actually, not voltage). It doesn't loose much of brightness even with half of nominal current and it also doesn't get that hot either. I measured ~52 celsius from the front plate of one led.

    Nice controller you have there! My circuit is controlled with 0-5V input also and has a adjustable reference point to adjust the point of full brightness. As the input voltage doesn't actually ever get to 5 volts (only 4.95 or so) so I've set the ref to 4.5V to be sure to get full PWM in a desired ambient light.

    Sorry for short answers, have to get back to work now..

  7. #17
    Maximum Bitrate bes51659's Avatar
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    Thanks for the picture. This is my LCD panel. So I just remove the metall tape, and carefully lift the back off?


  8. #18
    What can I say? I like serial. Curiosity's Avatar
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    That white stuff looks like glue?

    The ribbon cable looks like it's under that black tape. One thing to note is that the LCD panel is on the face, and most of the time attached to the metal frame. Or is that always? So you'll have to peel the black tape and flip the ribbon over and let it hang while you remove the plastic backplane.

  9. #19
    Maximum Bitrate bes51659's Avatar
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    Thank!
    Yes it is glue. I have glued the frame to my bezel.
    I have just email soumi, so the project is now on the go :-)

    Edit: The answer from Finland == I will make my own pcb.

    Hello!
    This company manufactures custom PCBs, designs electronic devices and
    sells electronic components. I cannot sent PCBs to you via email, you
    will have to tell me your address and what kind of PCB do you want
    As you may be aware, this company is in Finland, so if you are
    ordering from abroad, there might be additional expenses due to
    customs clearance and delivery.

    The very LED bar PCBs were designed and manufactured by ElectroForge,
    but we don't have them in stock. If you want to have similar PCBs, we
    can manufacture them for you. However, it is possible, that these PCBs
    won't fit into your display. The layout was specially designed to fit
    an LCD of the individual (Sniiki) who originally ordered the them, so
    it would be up to you to determine if it fits or not.

    Feel free to ask, if you have any questions.

    Best regards
    Niko Rolamo
    ElectroForge
    www.electroforge.fi

    P.S. Our website is unfortunately in Finnish only since we have little
    to none international sales at the moment.

  10. #20
    Maximum Bitrate bes51659's Avatar
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    Was looking for LED's and I reallize that your LED's seem to have outstanding performance! 1500mcd at 25mA!!!

    I am wondering a bit about whiteness. I have searched for LED's, and some say backlight for LCD but only one say backlight for full colour LCD! That one had 160mcd at 20mA.

    Do you think any 5600K LED would do, Sniiki? I would prefer right angled ones as I think that will be easier to fit. No?

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