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Thread: Controlling back lights directly?

  1. #31
    Maximum Bitrate Mickz's Avatar
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    FYI Looks like the High current version HT7939 can drive up to 39 White LEDs with a 5V input.
    GA-Z77N-WIFI, i5-2400S, 8GB, Intel 520 128GB SSD, M4-ATX Modified, 2 Rev Cams, 2 Web Cams
    8" 16:9 TRANSFLECTIVE, Win8-64, Dual GPS RX and Garmin PC + Odyssey Nav, FM-DAB+, BB-Rec
    T-Screen HVAC control, custom microcontrollers, code and FE. CarPC Project

  2. #32
    Maximum Bitrate Mickz's Avatar
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    Well, I’ve modified the unit for direct control of the LED backlight by the Vehicle dash-light PWM signal. The PWM control in this Vehicle is a 200HZ PWM square wave running at 10v Peak.

    Dash-light control full brightness: PWM stops and Line drops to 0 volts.
    Dash-light control 90% brightness: PWM around 10% on 90% off
    Dash-light control 50% brightness: PWM 50% (1:1) or 50% on 50% off
    Dash-light control fully dimmed: PWM around 90% on 10% off

    This is of course the opposite to what is needed to Drive the Enable/low freq PWM line. IE +5v is applied by the current LCD controller for full Brightness (Enabled) and 0V is off (Disabled).

    However, it is absolutely perfect for allowing me to keep the existing Enable control from the LCD PCB which is used to turn the Backlight off when no video is present.

    The modification required one NPN transistor to invert the waveform and 3 resistors. Two resistors are used to drop the 10v PWM down to the correct voltage to drive the transistor and the 3rd resistor (2k2) is used as the load resistor between the existing +5V Enable Line and the collector of the transistor. Almost no way to damage the LCD or controller this way as the only connection is via a 22k resistor.

    So, dash-lights OFF = no voltage to the transistor, so it’s switched off and as before, the Enable line is controlled by the LCD main PCB. Dash-lights ON and the PWM signal controls the Backlight, the Enable Line can still kill the Backlight in the absence of Video or in standby.

    I have left the existing backlight control from the LCD Main PCB as they work perfectly together.

    Just tried it in the Vehicle and its perfect!



    UPDATE:

    I posted some of this in my WebLog Build, also I carried out a few more tests in the Vehicle and the results are as follows:

    1. With LCD backlight controlled by the vehicle dash-light which is controlling the modified PWM input, AND also unsing the LED backlight 3 step logic built into the Monitor, the result is “almost” perfect in a pitch black location and no annoying GLOW effect.

    With only that control, normal driving with streetlights and other vehicles would be perfect.

    2. In complete darkness IE No street lights or vehicle headlights - night time country driving.

    With 1. above and the FE ALSO controlling the Windows brightness via the API, as most FE application seem to do, the extra reduction in brightness results in a perfect display, again without that annoying GLOW effect from my normally half lit backlight. I can only imagine how annoying a full lit backlight would be in an 8” screen that’s not far below eye level.

    Finally, if you have a monitor that doesn’t control LED Backlighting, and it’s seem there are a lot that don’t, then the effect will be even more dramatic. At least my unit dropped the backlight to 50% at night, but as I said, it was still a bit annoying in really dark driving conditions, now its perfect

    UPDATE:

    Just to clarify a few things here.

    I have been reading through some of the old threads on LED Backlighting control and the means used to attempt to control them. So here are a few points to consider when using this circuit.

    1. May already be inside your Monitor and only needs a simple mod for external control.

    2. The circuit can be controlled by either a PWM signal or a variable DC voltage.

    3. The voltage across the LEDs is DC when the IC is controlled with a DC voltage via the FB pin.

    4. The voltage across the LEDs is of course PWM when the IC is driven with a PWM signal via the Enable pin.

    5. Can be driven straight from a Microcontroller with PWM into the Enable pin.

    6. Parts don’t have to be SMD (except for the IC) but that is only a 6 pin device and the spacing is pretty reasonable, soldering with a fine tip iron would be relatively easy, if you have soldered before.

    7. No heat-sink required and very small size.

    8. Can be interfaced to the existing Vehicle PWM line with minimal effort and $1.60 in parts.

    Anyone having trouble understanding what’s being done here or wants a little clarification, please don’t be hesitant in asking, I’ll help wherever I can.


    Last edited by Mickz; 07-19-2012 at 08:58 PM.
    GA-Z77N-WIFI, i5-2400S, 8GB, Intel 520 128GB SSD, M4-ATX Modified, 2 Rev Cams, 2 Web Cams
    8" 16:9 TRANSFLECTIVE, Win8-64, Dual GPS RX and Garmin PC + Odyssey Nav, FM-DAB+, BB-Rec
    T-Screen HVAC control, custom microcontrollers, code and FE. CarPC Project

  3. #33
    Raw Wave
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    Has the latest reply or more been deleted from this thread, or am I really confused (ie, more than usual)? I'm thinking where the OP (or other) bit the PIC bullet and even posted the coding, and I'm certain it was good ol' Ryan_M... And I'm certain I replied, and it wasn't a PM, so unless my final/last reply got lost in transmission... or when I woke up!


    Anyhow, not that effects this reply....
    And not that I want to flog living horses...

    But FYI I found John Hewes' "Electronics Club" led page.
    When I saw the section "Avoid connecting LEDs in parallel! I thought of the single resistor OR current controller for multiple LED strings as discussed earlier herein.
    Though I merely gave the more obvious KIS problem when a string fails, that section mentions how LED voltages vary. (And as I just wrote elsewhere, unless recently overcome, even LEDs fabricated on/from the same silicon die will vary in voltage for a given current.)
    And that page's advice is merely for a common resistor which will compensate somewhat.... The effect is even more pronounced for a single Current Limiter because it does NOT compensate.

    Not that in this case that has much relevance now since I think using a current-limiter is a dead issue (it's simply not needed).
    But more for other projects etc, and as another link to some useful LED info.


    FYI - that "elsewhere reply" relates to car backlights - ie, stop/flasher/tail tail lights - using LEDs. The OP may consider using 4 ~3.5V LEDs in series without any resistor (especially if with a PWM for dimming, and initial flashing of the brake lights) instead of 3 with a series resistor.
    Last edited by OldSpark; 07-25-2012 at 04:27 AM.

  4. #34
    Constant Bitrate
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    I think I'm the only one who posted any code. I don't think any replies are missing.

  5. #35
    Newbie
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    Yeah I don't think any posts are missing either. I originally wanted to do this in the analog domain (since it's more what I know) and I wanted to avoid PICs since from trying to get into them years ago, I knew I had no time to learn assembly to program them. Since Dave pointed out that I can program them now in BASIC, I've decided to go that route. Learning curve is a lot less steep - and I might even remember a bit of it from when I was in school. Also this seems like an easy project to get my feet wet using the picaxe and something worth learning for future projects.

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