Ambient light sensing via LED response

Reinder de Haan rs at
Tue May 5 16:24:19 EDT 2009


C. Scott Ananian wrote:

> 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
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

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 :-(


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