Ambient light sensing via LED response

Rafael Enrique Ortiz Guerrero dirakx at
Tue May 5 18:44:05 EDT 2009

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.

(haven't checked the specs though..)

My two cents..;).

Rafael Ortiz

On Tue, May 5, 2009 at 3:24 PM, Reinder de Haan <rs at> 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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