Ambient light sensing via LED response

Tue May 5 22:52:04 EDT 2009

On Tue, May 5, 2009 at 8:34 PM, John Watlington <wad at laptop.org> wrote:

>
> 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.
>

i don't know if the measurements at sunlight would show the same
variations.. we would have to make new measurements, but for experience, the
variations of voltage regarding light sensing are not of considerable
amounts, so if the accuracy is 26mv, we would have to see if a perceptible
change in ambient light could be of a higher magnitude than 26mv, if not the
accuracy could be lost..

>
>
>  (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...
>
> Ok, thanks :).

>
>
>>
>> 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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>
>
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