Ambient light sensing via LED response

[John Watlington]

On May 5, 2009, at 6:44 PM, Rafael Enrique Ortiz Guerrero wrote:

> These measurements are really cool.
>
> But the question remains in whether the ADC could have a resolution  
> of 1mv?, i mean in light of these measurements is necessary to have  
> an ADC that can reliable sense these variations and then with that  
> basis  have a transfer function and add it to the algorithm.

Absolutely not.  The A/D is eight bits, with an input range spanning  
0 - 3.3V, so the best you
can hope for is about 13 mV per LSB.  I would guess actual accuracy  
to be closer to 26 mV.

But why do you say you would need 1 mV accuracy ?   Bright sunlight  
is far stronger than
the light sources he used.

> (haven't checked the specs though..)

Having the data sheet for the EC controller doesn't help --- 8 bits  
and recommended operating
voltage for the analog reference voltage is about all it provides.    
I had to ask a chinese speaker
to call the app. engineer to find out the input impedance...

> My two cents..;).
>
> Rafael Ortiz
>
>
> On Tue, May 5, 2009 at 3:24 PM, Reinder de Haan <rs at mveas.com> wrote:
> Hallo,
>
> C. Scott Ananian wrote:
> <snip>
>
> > A last resort would be hooking up a MOSFET as a simple amplifier --
> > again, you're not worried about linearity or any such niceties, but
> > you'd still need a good match for your MOSFET's threshold voltage...
> > some real measurements to replace the WAGes would go a long way.
> >   --scott
> >
>
> measured on a B1 XO1 laptop (where the leds and the series resistor  
> are
> wired in parallel it seam) :
>
> almost dark: 0mv
> ~3meter away from a 8w PL; bare led: ~2mv
> ~50 cm below ~25W halogen desk lamp; bare led: ~40mv
> bright white led directly on bare led ~200mv
> bright white led directly on light guide of the bat.led (lcd side)  
> ~50mv
> ~50 cm below ~25w halogen desk lamp (~75* angle to the axis of the  
> light
> guide) ~5mv
>
> i measure this between  GND of the laptop the led side of the series
> resistor.
> all leds seam to be about the same.. i did not compare the different
> light guides.
> the main battery and DC power where removed, the RTC baterry was still
> in place.
>
>
> the meter i measured this with was fixed in the 2000mv range
> and was abou 10Mohm when i connected it to another meter in resistance
> mode; the (volt) meter read 250mv at that time.
>
> adding a 10Mohm resistor across the meter halved the
> ~50 cm below ~25W halogen desk lamp; bare led: ~40mv reading to
> ~20mv
>
> strangely enough when i add that resistor in series the meter said  
> ~60mv
> so voltage on the leds must have been ~120mv? i will have to  
> investigate
> this at a later time...
>
> if you want more measurement doen feel free to ask.. i also have a XO1
> (production model ?C1?) that i could measure.
>
> i dont have a lux meter :-(
>
> Greetings,
> Reinder
>
>
>
>
>
>
>
>
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