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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
    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, 07:01 PM.
    My OLD 2001 Mitsubishi Eclipse GT:
    "The Project That Never Ended, until it did"


    next project? subaru brz
    carpc undecided

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    • #3
      And for a possible PWM circuit....

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      • #4
        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..
        My OLD 2001 Mitsubishi Eclipse GT:
        "The Project That Never Ended, until it did"


        next project? subaru brz
        carpc undecided

        Comment


        • #5
          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, 06:55 AM.

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          • #6
            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.
            My OLD 2001 Mitsubishi Eclipse GT:
            "The Project That Never Ended, until it did"


            next project? subaru brz
            carpc undecided

            Comment


            • #7
              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).

              Comment


              • #8
                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.

                Comment


                • #9
                  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..
                  My OLD 2001 Mitsubishi Eclipse GT:
                  "The Project That Never Ended, until it did"


                  next project? subaru brz
                  carpc undecided

                  Comment


                  • #10
                    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.

                    Comment


                    • #11
                      I agree, OldSpark, that LED's can handle more current than rated if the duty cycle is appropriate. But if you don't get your duty cycle right you risk blowing the LED's.
                      In our situations it's just as easy to create a current regulated supply that provides the correct current for 100% duty cycle.
                      And for me the CAT4101 makes it dead simple. The parts values are not critical and you just dial in the current you want.
                      Couple that with a PIC and you get a very flexible widget.

                      I would also argue the statement that the CAT4101 is for a single string or high power LED's.
                      If all the led's are working, multiple strings or a single string are the same. Right? (he humbly asks as he has no actual electrical training)
                      And since it can be adjusted, it can drive anything up to 1amp.

                      In an ideal world each series group of LED's would be driven separately, like this (see pic)
                      But the display manufactures don't do it that way (at least the ones I've seen). They simply have multiple series strings paralled together.
                      The best you can do for that is regulate the total amount of current supplied. Regardless of the current supplied, if one LED dies it puts the rest of them jeopardy.
                      Attached Files
                      Last edited by davekra; 07-10-2012, 07:06 AM.

                      Comment


                      • #12
                        Hmm, I think we've scared off the OP.
                        This is a good discussion and it shows there are many ways to accomplish something.
                        It's also good info to search for but where is the OP. We need your input if you'd like us to guide you to a solution that's right for you.

                        Comment


                        • #13
                          Originally posted by OldSpark View Post
                          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.)
                          that cat4101 driver is actually a very capable driver, the only reason i recommended against it is the form factor can be a little daunting to someone who hasn't really dealt with smd components before.

                          there are actually a lot of drivers like that one-- all of them have a max rating, but use a specific resistor connected to a sensing pin to set the maximum current that is allowed to flow through the layout. so while the driver is capable of outputting 1000mA, you can set it to only output 20mA.

                          Originally posted by OldSpark View Post
                          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.
                          name brand led's are capable of handing peaks like that. no-name led's are much less forgiving in all out-of-spec regards. but i don't believe that cheap led's were ever used in these backlights, as the color consistency is very important, and cheap led's do not have that type of consistency. they are probably using cree, nicha, or philips led's for it.


                          Originally posted by davekra View Post
                          I agree, OldSpark, that LED's can handle more current than rated if the duty cycle is appropriate. But if you don't get your duty cycle right you risk blowing the LED's.
                          In our situations it's just as easy to create a current regulated supply that provides the correct current for 100% duty cycle.
                          And for me the CAT4101 makes it dead simple. The parts values are not critical and you just dial in the current you want.
                          Couple that with a PIC and you get a very flexible widget.
                          completely agreed. i really like the low parts count required for it as well!

                          Originally posted by davekra View Post
                          I would also argue the statement that the CAT4101 is for a single string or high power LED's.
                          If all the led's are working, multiple strings or a single string are the same. Right? (he humbly asks as he has no actual electrical training)
                          And since it can be adjusted, it can drive anything up to 1amp.
                          correct. (lucky for you, i've been building/designing led lighting stuff for my car for the last 2 years. the led section at hidplanet.com is invaluable ) the current is just split between each parallel strings. you gain light density at the cost of current control accuracy. when you have multiple strings, different strings can heat up differently, causing each string to slightly change how much current it pulls.

                          in the case of high power led's, it is preferable to mount them all on the same heatsink so each parallel string is subjected to similar heat amounts, so that fV(forward voltage--each diode has it, and as it heats up, it lowers) variance is kept to a minimum.

                          in the case of low-power led's(i believe that is typically anything below 75-100mA), they don't output much heat, so it's not as much of a problem.
                          you really can voltage regulate them without any problems, especially name-brand led's. it is best to current limit each series string with a resistor in that case.

                          Originally posted by davekra View Post
                          In an ideal world each series group of LED's would be driven separately, like this (see pic)
                          But the display manufactures don't do it that way (at least the ones I've seen).They simply have multiple series strings paralled together. The best you can do for that is regulate the total amount of current supplied.
                          most uses of low-power led's don't use any sort of current regulation, many times, they just use voltage regulation. you can really regulate either the voltage or the current-- as long as one part of the equation doesn't change, the other part is constricted enough to not change enough to damage anything.

                          Originally posted by davekra View Post
                          Regardless of the current supplied, if one LED dies it puts the rest of them jeopardy.
                          what you're referring to is a cascade failure(led heats up, the fV lowers, allowing it to consume more power, which in turn heats it up more, this keeps repeating until the amount of current passing through the led exceeds it's mechanical limits and the led burns up)-- that is another thing that most common in high power led's. it's not to say that low power led's can't have it happen--any diode is susceptible to it, but because their power consumption is so low compared to the large amount of copper pad area they typically are connected to, that helps to radiate any heat they create, it can only become a issue when they are pushed well past their limits.

                          with low power leds, using voltage regulation would not affect anything if a led died- for instance, i use 12v-2A regulators in my car--one for each led board. most of my arrays consume less then 1A each, more typically in the range of 100-500mA per board. if a led dies in a array, yes, it does make more power available to the other led's, but it is excess added to excess, so while the dead led opened up space, the 12v regulation restricted everything to not allow any of the other strings to consume any more power then they were already consuming.

                          clear as mud?
                          My OLD 2001 Mitsubishi Eclipse GT:
                          "The Project That Never Ended, until it did"


                          next project? subaru brz
                          carpc undecided

                          Comment


                          • #14
                            Originally posted by davekra View Post
                            I would also argue the statement that the CAT4101 is for a single string or high power LED's.
                            If all the led's are working, multiple strings or a single string are the same. Right?
                            Alas if one string fails, the rest divide the current and pop! Ergo, no point having a sophisticated LED current controller.
                            They are intended for single-string high-current LEDs which are more critical wrt to current (LEDs are current devices, not voltage).


                            PWM is unaffected by the number of (failed) strings.
                            If resistors or current limiting is to be used, then it should be one resistor per string.
                            The only "reliable in usual practice" designs that use a common resistor for multiple strings is where there are several strings - eg, if 10 strings and one blows, then the others get 10% higher current. And with low-current (20mA) LEDs, that is usually not a significant problem.

                            The PWM limiting instead of a resistor or current limiter is used for high voltages where power conservation is desirable. IE - say a 12V string off a 24V or 110V or 360V supply etc, the resistor or current limiter would waste (24-12) or (110-12) or (360-12) compared to the 12 used by the LEDs. (Times the current - I'm just indicating the proportions, ie, 1/2 or ~8x (98/12) or ~30x (348/12) the power consumed by the LEDs.)


                            If current limiters are required, I'd be tempted to use the LM317 voltage regulators as only one resistor is required and they are self protecting (thermally). Just pick the appropriate type to handle the current and heat (or fit a heatsink). IMO that's probably simpler than two-transistor current limiters etc.
                            But current limiters are usually overkill, especially since one per string is required.

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                            • #15
                              Originally posted by OldSpark View Post
                              Alas if one string fails, the rest divide the current and pop! Ergo, no point having a sophisticated LED current controller.
                              They are intended for single-string high-current LEDs which are more critical wrt to current (LEDs are current devices, not voltage).
                              you make it sound like failure is guaranteed with multiple strings-- it's not. and while led's are current devices, part of current is voltage, so voltage regulation does limit the current. current control is more preferable because it more tightly controls the led, but low power led's like what are in the screens talked about here are fine with voltage-only regulation, as the heat that the led's produce is easily accounted for by the copper on the circuit boards.

                              the only way to have a cascade failure happen is to overheat the P/N junction of the led. it is very difficult to do this on low power led's when taking all the design considerations to mind.

                              i've been running my led tail lights now for almost 2 years with voltage regulation only... but i designed the boards correctly for the situation..

                              Originally posted by OldSpark View Post
                              The PWM limiting instead of a resistor or current limiter is used for high voltages where power conservation is desirable. IE - say a 12V string off a 24V or 110V or 360V supply etc, the resistor or current limiter would waste (24-12) or (110-12) or (360-12) compared to the 12 used by the LEDs. (Times the current - I'm just indicating the proportions, ie, 1/2 or ~8x (98/12) or ~30x (348/12) the power consumed by the LEDs.)
                              i don't think you fully understand the drivers being talked about-- the pwm input feature on these drivers is only a on/off switch for the regulator-- it doesn't tell the regulator to do anything but turn on and off.

                              the advantage to this over the LM series regulators is that it needs less parts to be current regulated and have pwm capabilities. this will accept a raw pwm input, where the LM regulators would require a transistor on the output to utilize the pwm signal.

                              can the lm-series be current regulated? you bet. i even recommended that method above simply because they are a larger regulator, so they are a little easier to work with...
                              My OLD 2001 Mitsubishi Eclipse GT:
                              "The Project That Never Ended, until it did"


                              next project? subaru brz
                              carpc undecided

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