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Thread: Auto DIM LED monitor when turning car illumination on?

  1. #1
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    Auto DIM LED monitor when turning car illumination on?

    Hi, I was wondering if there was a simple hack that can be done on a LED monitor to dim the LED a little when turning the car lights on?
    In brief, I notice 2 visible wires (red/black) going from the circuit board to the inside of my lilliput monitor. I assume those are the wires to feed the LED's in the monitor?
    My idea is something like a basic relay that would change the voltage when a current is sent. Something like where the relay coil would be connected to the car 12v illumination wire, then the red wire of the monitor could be spliced where com would go from the source of the board, then NC would go the monitor directly and NO would have something infront of the wire to reduce the voltage?

    I know very little in electrical engineering lol! This is just some wacky idea....? But I wonder if anyone with more experience in this stuff can shed some light if it's do-able?

  2. #2
    FLAC mauri's Avatar
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    Power the led with one little uP and PWM mode...

  3. #3
    Variable Bitrate
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    i agree with mauri. IF you know what you are doing, and, you are able to hack open the lcd, then, re-wiring the back light to work , from a separate PSU, that is yours to controll, is the "easiest" method...

  4. #4
    FLAC mauri's Avatar
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    If you're not an "expert" electronics \ hardware design and do not know how to use the uP, you may very well generate a PWM output with a simple NE555.
    https://www.google.it/search?q=PWM+w...iw=933&bih=424

    Alternatively still an Arduino ....

  5. #5
    Raw Wave
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    If it's a simple 2-step brightness you want, you should be able insert a series resistor. That could be shorted out for full brightness - probably using the NC (normally closed) contact #87a and common #30 of an SPDT relay (across the relay). The light's +12V energises the relay hence opening the contacts so the LED current flows thru the resistor.

    The resistor value would be found by experimentation.


    There are other methods of current limiting but IMO PWM is overkill - that's only used when you want variable and proportional or linear dimming (ie, proportional to incandescents or the turn of a knob). Plus you'd have to switch in the PWM unless you want it running all the time.

  6. #6
    Variable Bitrate
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    i liked the idea, so i went on to try it, (with a spare broken led backlight lcd i had). i found out, that the backlight itself :
    1) had no current limiting ressistors (normal, since current is supposed to be monitored by the lcd module)
    2) even though the lcd required 14.5v and 5v to work, the backlight ,was barly On with 20v... so, if your screen is like mine, you might need, a switching power supply... (unless ofc, you take the outpout of the lcd controller, and use it as input to your pwm or whatever else method )

  7. #7
    Raw Wave
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    So you add a resistor or current limiter (LM317 & resistor or 2 transistors & 2 resistors) in either end of the backlight LEDs.

  8. #8
    Variable Bitrate
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    so, i looked a bit more, on my spare LCD screen (A broken one)... the connector of the led backlight looks like that :
    Name:  IMG_20140326_210224.jpg
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    so as you can see, it is not so easy to just "interrupt the VCC line"
    what i was thinking was : use the spare controller, to cut out the part with the lcd controller, and build a completly new "led driver" with that part of the board as the connector...
    do you think that the rest of the lcd controller, will have a problem working, if it detects that there is no "backlight" connected???
    the driver has 40 leds, the connector, has (it has more, but uses only those) 1wire for Vcc, and 5 for grounds, so it has 8leds connected in serries, (with 5 parallel rows). each row , requires about 25v to work.

    so if i wanted to do, what you suggest oldSpark, i would do it like this ? :
    Name:  LedBackligt.jpg
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    (where each led, counts as 8).
    the resistors R, are there for current limiting. normally there is 25v going to the backlight (or whatever volts are actually required), but when the external signal arrives, the vcc, changes path (with the relay) , and has to pass through the Potentiometer, which then can be trimmed, to limit the TOTAL current, down to whatever you desire the low brightness to be ??

  9. #9
    Raw Wave
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    Yes, except do as I described - insert the resistor in the +V line. Then connect the relay contact across the resistor (SPDT; 87a & 30).
    That way there is no break to the LEDs when switching.

    Assuming 20mA LEDs & hence 100mA total and a 20% voltage drop, I'd suggest a 27R resistor minimum, or a 100R trimpot (else 200R, or 100R fixed plus 100R trim etc).
    1/4W should be ok but use 1/2W resistors.

  10. #10
    Variable Bitrate
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    cool, thanks! although i would certainly prefer to just interrupt the Vline that the lcd board itself outputs... but that ribbon cable, cant be messed with >_> (or can it ? :P )

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