CCFL backlight replaced with white LEDs

jornada backlight

[Jeroen Domburg], one of our favorite hackers, has posted how to replace a cold cathode based backlight with white LEDs. He had recently purchased an HP Jornada 680 on eBay and the backlight was looking very pinkish. The Jornada has a QWERTY keyboard, CF slot, PCMCIA slot and can run Linux. Sounds like a pretty decent mini-terminal (now that we posted it you won’t be able to find one cheap though). In the space where the backlight was he was able to place 14 white LEDs. To get the  40V necessary to drive the series he built a boost-converter using an ATTiny13. The new backlight is just as bright and uses a 1/3 less power than the original. Half the Jornada’s power was being used by the original backlight, so this mod should give a decent increase in battery life as well.

Comments

  1. anon says:

    FUNCTIONAL, FIXES A PROBLEM, AND INCREASES BATTERY LIFE, NICE HACK

  2. yeish! Couldnt you just put the leds in parallel and keep from needing to do the voltage amp?

    This is rather interesting though. Wonder if other companies would do this.

  3. Liam says:

    Hmmmmmmmmmm

    People have been doing this on laptops for some time now, particularly when the old ccfl’s have died or just gone dim. I’ve been thinking about it myself, but uniformity has always been a concern. This article has been useful to me in that I can see quite clearly in these pictures that the illumination is quite unevenly distributed, and would definately be no good for my laptop.

    Yes, I’ve thought about using diffusers etc. but my feeling is that once it starts getting this complex, it’s not worth doing. Plus the absorption from the diffuser will dim the display.

    Very good hack for a little portable, though!

  4. you could always use low-profile, wide-angle LED and put some kind of defuser on top… it would completely fix uniformity and you’d loose a little brightnight but I think it’d be a heck of a lot more uniform then what’s there.

  5. Antiwhack says:

    2. if you put the led’s in parallel, you’d be dividing the current, which is what drives the brightness in an led, by keeping them in series, you’re keeping a consistant current and you’re just dividing voltage.

  6. dioxide says:

    hal (#2), you don’t run diodes in parallel. unless the diodes are all completely identical, it’s too common that one of them will end up with all of the current, and blow. then the rest of the diodes have to deal with that much more current, and another blows, etc etc.

  7. freiheit says:

    What would happen if I used coloured LEDs?

    It would be cool if there was a specific reason I wanted a tinted scre….!!!

    Like using red LEDS when the laptop will be used at night to preserve night vision??

  8. Kowalski says:

    awesome! i have one of these, a jornada 720 wiah a wifi pcmica and 1gb storage sf… had a bit better spec than the 680, uses ARM, so i can use a lot more progs because of the more common architechure, and supposedly can display more colors. for more on these go to http://www.hpcfactor.com, a very nice forum for all hpc-based devices, including jornada’s

    my windows ce collection, btw- inchuding the 720

  9. usbcd36 says:

    LED backlighting is nothing new (not to say this isn’t neat). Ben Heckendorn has modified PSone LCDs and pocket TVs for use in his portables (they’re all on hackaday in various places), and he even bought factory-made LED backlit LCDs for his Atari Phoenixes.

  10. IMWeasel says:

    You can put the diodes in parallel if you put a limiting resitor in series with each one.

    Another way is to put a few diodes and a resistor in series, so that the sum of the forward drops is a bit less than the battery voltage. Then put these groups in parallel. This wastes less power in the limiting resistors and still keeps the voltage needed down below the supply voltage.

    I realize that he said he wanted to build this with parts he had on hand at the time, but the best, and most efficent way, is to use one of the fancy new white led driver chips like this:

    http://www.national.com/news/item/0,1735,726,00.html

  11. Alexander says:

    I wonder if this would work with my Libretto… Hmm…

  12. nguyen_van_thoc says:

    #7 – I assume you’re joking, but just in case – how about using a red theme and dimming your backlight? But of course that takes away your excuse to hack on your equipment…

  13. Zenaida says:

    Seems like the thing to do would be to make a grid of the LED’s, so you can turn just one on at a time, then cycle through them all quickly enough so that you don’t notice any flicker. That way you only burn 20mA. Since he used an MCU anyway, it doesn’t seem like it would be too much harder. Although I guess you need 8 or so output pins to do that, which the ATTiny might not have.

  14. p00nlog says:

    there’s lots of other devices that run some other form of *nix as well. i have an nec mobilepro 790 which is extremely similar in terms of functionality and size. this model and similar are dirt cheap on ebay. it has a cf slot, pcmcia, half vga touchscreen, touch-typeable keyboard, etc. netbsd hpc-mips runs on it like a dream, including x. just something to keep in mind looking at devices like this, jordanas aren’t the only line.

  15. IMWeasel says:

    “That way you only burn 20mA.”

    LEDs don’t work that way. Yes, you can pulse them, but the average current has to be approximately the same to perceive the same brightness. The only reason LEDs are multiplexed is to reduce the amount of control circuitry, not to save power.

  16. daniel says:

    “LEDs don’t work that way. Yes, you can pulse them, but the average current has to be approximately the same to perceive the same brightness.”

    Actually, I believe studies have shown that the brightness does not tend to fall of linearly with duty cycle (that is, 50% dut = 50% brightness) but closer to logarithmically because of persistence of vision.

  17. neato says:

    I like it! but how about making use of the micro’s comparator and pwm instead of the crusty (read: slow) adc! you can use the interrupt (positive edge) to turn on the pwm output and negative to turn it off again. http://www.edn.com/article/CA6330096.html?spacedesc=designIdeas&industryid=44217

  18. Alexander says:

    Would it be possible to have a list of ultra-portable palmtop/handtop/PDA devices that run *nix? I got the Toshiba Libertto, this one on this page, the NEC Mobilepro 790 and that’s it.

    What I’m looking for is something to become my alarm clock (streaming MP3 through wireless) and my small portable to keep near my bedside when I have an idea in the middle of the night. Currently I have a Toshiba E740 and it’s really starting to annoy me as every time I set it down it hard-resets and loses all my data that I don’t save to my CF Card.

    Thanks for any help.

  19. p00nlog says:

    http://www.netbsd.org/Ports/hpcmips/#supported

    this is the site for the netbsd-hpcmips distribution (what runs on the mobilepro). huge list of supported devices and to what extent they are supported. of course, some of these devices are rather obscure, and you’d have to do some googling to get the finer points, but a good starting point at least.

  20. IMWeasel says:

    “Actually, I believe studies have shown that the brightness does not tend to fall of linearly with duty cycle (that is, 50% dut = 50% brightness) but closer to logarithmically because of persistence of vision.”

    Do you have any references to these studies? I’ve had this debate with people before and nobody has ever been able to come up with any. I personally think it is a myth.

    I have designed several multiplexed LED displays, and my own personal experience is that it is reasonabley close to linear. I’ve pushed it as far as a 10:1 multiplexing and in that case I did have to run 10 times the current to get the same brightness.

  21. Justin says:

    I would love to find someone with one of these or something like it broken so i could play with it

  22. daniel says:

    “Do you have any references to these studies?”

    Unfortunately I can locate none at the moment. My main reason for believing such is my own experience when I replicated a test with a 10 segment LED bar graph, with the segments driven in 10% duty increments, and to my eye the gradations were closer to logarithmic (I had printouts of a linear and logarithmic scale, both in 10 gradations to match to). The phenomenon may be due to some other factor but I have seen it in practice.

    A photograph of the experiment showed a linear scale, but this was not as it appears to the eye, probably due to some process in the eye or the brain in processing the image.

  23. IMWeasel says:

    “My main reason for believing such is my own experience”

    You are the first to disagree with me and claim actual experience. In the past it’s always been a “well, I read it on the net somewhere” or “so and so told me” kind of thing.

    So now I think I am going to have to build something to test this now. Like maybe take a microcontroller and program it to PWM a LED at different duty cycles, and then build a constant current driver and adjust it to match the brightness and then measure the current and compare to the PWM duty cycle.

    If I really wanted to get fancy, I could make the microcontroller control both the PWM and the constant current source and be able to switch which goes to which LED and change which LED was being adjusted. The microcontroller could be programmed to do a sort of double blind test that way because I wouldn’t know which LED was constant current and which LED was PWMed when I was matching the brightness. So many projects to do, so little time. :)

  24. adean says:

    I’ve had this argument before too. AFAIK the eye responds linearly to the total power impingent on it (if that is even a word) – unless – it is slow enough to visibly flicker. In that case it will appear brighter because of the retina’s movement detection circuitry.

    BTW I would like to try RGB LED backlighting to enhance the colour gamut of an LCD, apparently (as in I am guessing) this is how the high end monitors do it.

  25. d1 says:

    I got a 3 year old FOSA NB-34B laptop with a blown inverter, near-dead battery and serious paint fade and i wonder if i can do this to it. The cold cathode still works but the inverter has blackened parts. I was *theorizing* that by getting 11V from the Pow Sup. it will go to a resistor, stepping it down to 4V, going to a variable resistor (Bright Control) to an array of White LEDs. I know that this theory (excuse me) sucks because the technicians laughed, so any brighter ideas? Anyone?

  26. seth says:

    Has anyone tried a single, ultrabright LED with a piece of perspex rod? You can get the square or triangular rods (clear or diffused). I’m thinking if you cut it right you could get a nice distribution of light with just the single LED..

  27. Vincent says:

    do you get any longer run time running off the battery?

  28. Cyrus says:

    Why are the “Y” and “Z” keys on your keyboard switched? Did anyone else notice this?

  29. twentynine says:

    Because he speaks/types German, maybe? How did you notice two tiny keys and not a screen full of German text? ;)

  30. Jerry S says:

    The brightness of DLP tvs is by duty cycle modulation and the red, blue, and green colors appear sequentially, so the response of the eye and brain to pulsed light and color has been well studied. No need to do it over again.

  31. Old Comedywriter says:

    I bought a bad no-picture 15″ HDTV on Ebay for $50, a LED flex strip light for $10, and now have a really cool (literally) flat panel TV. Whites are a little dim, slight “stage lighting” effect, but it works fine with the right color temp setting. Took only an hour to do the mod. Now it sits next to my repaired $50 21″ computer monitor with all new electrolytics and $50 Ebay P4 computer, which is a server for my free salvaged weather station (also with LED replacement backlight.) The fun I had in repairing all this: Priceless.

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