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Thread: Controlling back lights directly?

  1. #1
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    Controlling back lights directly?

    I've been trying to find solutions for controlling lcd backlighting directly. I see there is some software solutions but if it doesn't set the LED brighness directly then it's not really what I want - even a 'black' LCD display dives off a lot of light in the dark. Everything I've found so far consists arduino's or fusion brains to custom pic circuits. It's gotta be easier than this. AFAICT all of the touch screens I'm considering using are LED back lit. Couldn't I just power the LED's directly using a simple circuit that varies the duty cycle based on either the instrument cluster lighting or a pot as an input?

    Which brings me to another question. I need a display that I can get at the LED wiring. One of the displays I'm considering is the Lilliput 669GL-70NP/C/T-HB-RV. I've been able to find from pictures that there is a 4 and a 2x30 conductor ribbon cable connecting the display to the controller board, but there is also a two wire connection as well. I've been told that this display powers the LEDs through a single jack - is this it? I can't find any technical docs to figure this out for myself so can anyone confirm this?

    Thanks!

  2. #2
    MySQL Error soundman98's Avatar
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    i just so happen to have a 629 led and a 669 led sitting around... so i took them apart a little..

    629:
    this is the driver circuit, the IC is labeled IL68G, but i can't find that number in any IC database-- i was hoping to find a 'enable' pin on it to make pwm really easy, but no..




    669:
    didn't open it up yet to look at the driver, but it is just that large white connector with the pink and white wires coming out.





    so either is pretty easy to overtake the led's, but no easy way to add pwm to what is already there...

    i'll open up the 669 completely next and see if there is anything worthwhile in there..

    edit: nope.. this one uses a different chip with a smaller external component count, but is the same form factor. it's labeled5126R, but i couldn't find anything on it either..

    pics of the controller anyways:


    and the connector:
    Last edited by soundman98; 07-07-2012 at 07:01 PM.

  3. #3
    Raw Wave
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    And for a possible PWM circuit....


  4. #4
    MySQL Error soundman98's Avatar
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    you should still use a transistor on the output because, if i remember right, pin 3 can only sink 200mA max. the led's they use 'shouldn't' be above that, but there is really no way to know, so i'd rather blow a external transistor instead of a entire 555 timer..

  5. #5
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    OldSpark gives you the PWM circuit...then use that to drive a CAT4101 LED driver.
    http://www.digikey.com/product-detai...5CT-ND/1933886

    Personally what I'd do, and what I did, was use a PICaxe controller to drive the CAT4101.
    I'm using a light detecting resistor, but you could find a way to read your instrument lighting.
    I found the LDR to work well. The display is constantly adjusted to match the ambient, but at a slow change rate so you don't notice.

    This thread has more info:
    http://www.mp3car.com/lcd-display/15...01-np-c-t.html

    This is another good thread:
    http://www.mp3car.com/lcd-display/15...backlight.html

    "Getting at the LED wires"...If the LED's have actual wires you're golden. Soundman's first pic shows the two conductor ribbon cable for the LED's. This is what I had. Don't unsolder the ribbon cable though. Cut the traces on the circuit board a little bit away from the ribbon cable, clean the green mask off and solder to the bare traces.
    Last edited by davekra; 07-09-2012 at 06:55 AM.

  6. #6
    MySQL Error soundman98's Avatar
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    Quote Originally Posted by davekra View Post
    Don't unsolder the ribbon cable though. Cut the traces on the circuit board a little bit away from the ribbon cable, clean the green mask off and solder to the bare traces.
    +1
    i was going to mention exactly that. that way if you ever need to turn it back to the way it came for some reason, all you need is some small jumpers to complete the old traces, and you're done.

  7. #7
    Raw Wave
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    My bad! I intended the diagram with a MOSFET etc. Easiest is an N-channel with the Gate fed via resistor of at least 82 Ohms from 555 pin-3 (only to protect the 555 output) with the Drain to the LED- and Source to GND. (And maybe a 1M resistor between G & S to ensure no false triggering if the 555 output is high-impedance.)


    PICAXES will also do it and have the advantage of lower power consumption (an idling 555 is 10mA) as well as the ability for novel inputs (buttons, light sensors, etc).

  8. #8
    Constant Bitrate
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    I really would push for the CAT4101 over a FET or transistor with a current limiting resistor. Especially if this is driven from unregulated 12volts and if the person has limited experience (like me).
    A variable resistor is used to dial in the exact current needed but it's not handling the actual current.

    I know you can drive the LED's with more current than recommended IF you adjust the PWM to something other than 100%, but having the whole range of PWM, from 20% to 100% is nice.
    It also means that if you go to 100% duty cycle you won't blow the LED's.

  9. #9
    MySQL Error soundman98's Avatar
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    considering that, it might actually be better to use a LM-series adjustable regulator, as long as the dropout voltage still works at the required input-- all those CAT401 series drivers are smd, so they can be a little daunting for a first-timer..

  10. #10
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    IMO the concern about over-current for LEDs is overrated. It isn't that critical - it's more of a life-span issue.

    And drivers like the CAT4101 are for a single string AND high intensity LEDs (ie, up to 1A) which I doubt backlights use. (They are probably multiple strings of 20mA LEDs - not that I am up with the latest in backlighting, but high-current LEDs (CREE etc) are usually NOT suitable for backlighting.)


    PWM circuits can be maxed out to some value - eg, 20%. That's a technique often used for high-voltage supplies without current limiting resistors.

    And keep in mind that even a 20mA LED may handle peaks of 200mA etc - eg. for 1% duty cycles and not exceeding 1mS. I'm sure even previous doubter bes51659 will back me up on that.

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