light sensor

Discussion in 'General Electronics Chat' started by bug13, Nov 16, 2015.

  1. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Hi guys

    I don't have much experience with light sensor. So I want to detect light and dim my LCD screen accordingly (very dim at night and full brightness in day time)

    • Will and LDR good enough? Or do I need an photo-diode with an current amp?
    • What specs should I be looking for?

    Thanks guys
     
  2. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    LCD screen in a laptop,tablet, or what? Is the backlight CCFL or LED?

    ak
     
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    You can use a simple LDR or even a Red LED can act as a light sensor. You need high impedance but it is straightforward. You need a JFET op amp.


    View attachment 94805
     
  4. DickCappels

    Moderator

    Aug 21, 2008
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    You might want to obtain the schematic of the LCD you wish to modify. The backlight power supply will be of particular interest and it probably has a digital control input -the protocol being another possible piece of the puzzle.

    You might find some of the ideas in the following patent applications and grants to be useful.

    US 20020118182 A1
    US 9129549 B2
    US 9070317 B2
     
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  5. ian field

    Distinguished Member

    Oct 27, 2012
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    With a bog standard IR photo diode - a bog standard LM358 is sufficient for the high input impedance voltage follower.

    I could post the hand trace I have from an outdoor night light, but it includes a transformerless supply, so I can't do that till I get around to editing it.
     
  6. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    It's just and typical low power (5v) graphical LCD with touch capability. Its nothing high voltage if that's what you are worrying about. I just need to sense the light, and feed a PWM signal to the control pin to change the brightness.
     
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  7. ian field

    Distinguished Member

    Oct 27, 2012
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    Some of the older ones; the brightness control was just a slider pot feeding a DC control voltage.

    Its worth checking out whether you actually need PWM.
     
  8. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    I can't see your attachment.
     
  9. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    I'll redo it tonight.
     
  10. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    I always know that perceived brightness is not linear with luminance, but good to see some numbers and some equations. in patent US 20020118182 A1, Quote:
    Too bad I can't see the picture of the other two patent, but I also found patent US 9129549 B2 is useful for me in term of the characteristic of using photo diode and amplifier to sense ambient light. And patent US 9070317 B2 is too advance for my current, but maybe useful for future project.

    Thanks a lot.
     
    Last edited: Nov 17, 2015
  11. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    For some reason, after I edited my last replay, I lose some of my post. Here is the missing part:

    Thank you for the patent you listed, I found them very useful for my need. Thanks a lot!
     
  12. GopherT

    AAC Fanatic!

    Nov 23, 2012
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