light sensor

Thread Starter

bug13

Joined Feb 13, 2012
1,863
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
 

DickCappels

Joined Aug 21, 2008
6,544
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
 

ian field

Joined Oct 27, 2012
6,539
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
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.
 

Thread Starter

bug13

Joined Feb 13, 2012
1,863
LCD screen in a laptop,tablet, or what? Is the backlight CCFL or LED?

ak
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.
 

ian field

Joined Oct 27, 2012
6,539
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.
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.
 

Thread Starter

bug13

Joined Feb 13, 2012
1,863
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:
In addition, brightness adjustments that are exponential in nature provide less luminance change at lower display luminance levels and more luminance change at higher display luminance levels. The logarithmic nature of the human eye perceives equal luminance step ratios as equal brightness steps. To a user, a luminance change from about 10 Nits to 12 Nits (a ratio of about 1.2) appears like the same luminance change from about 100 Nits to 120 Nits (a ratio of about 1.2). A Nit is a unit of luminance for light reflected, transmitted, or emitted by a surface, such as a display panel. Brightness adjustments that do not correspond to the perception capability of the eye often result in different brightness levels than what is needed or desired. For example, a display device may have a daytime brightness range from 50 to 450 Nits. A brightness control system which linearly increases the brightness as a function of a control device such as a potentiometer or brightness step controller would be too sensitive for low brightness levels and not sensitive enough for the higher brightness levels. This brightness range may have about 8 steps, with each step increasing the brightness by about 50 Nits.
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:

Thread Starter

bug13

Joined Feb 13, 2012
1,863
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
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!
 
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