[Fwd: Re: Ambient light sensing via LED response]
Reinder E.N. de Haan
reinder at mveas.com
Wed May 6 18:03:21 EDT 2009
-------- Original Message --------
Subject: Re: Ambient light sensing via LED response
Date: Thu, 07 May 2009 00:02:49 +0200
From: Reinder de Haan <rs at mveas.com>
To: rafael at laptop.org
References: <200904290017.n3T0HL2v006886 at new.toad.com>
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Rafael Enrique Ortiz Guerrero wrote:
>
>
>
> On Tue, May 5, 2009 at 8:34 PM, John Watlington <wad at laptop.org
> <mailto: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..
>
>
i did some more experiments and at ~20 cm from the halogen lamp i doest
matter is i turn the backlight full on of off .. i dont have color
anymore....
at that distance the bare led gave about 250mv (maybe a bit more into a
very high R fet gate...)
i guess it would be around 70mv whit the light guide in front of it..
(i guess this be more (150mv?) with a very high R (~25 Mohm?) fet gate
as only load)
i would add a simple amp (mosfet? or maybe cheap opamp ?)
to amplify ~X10 so 300mv would be full scale..
my memory is dusty but that should be posible with 2 resistors an a fet
? gain would be posible to change by replacing one of the resistors with
a bigger / lower value...
maybe a trasistor is better bc the leds is acts as a current source ?
ideas welcome
if i can get the partnr of a fet/transistor that would be usable (and
cheap enough) i could maybe order a couple of them from say farnell and
hack something together here....
is there a free or hackable input on the current EC ? (or is that
different from the ec that will be in 1.5 ?
> (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
> <mailto: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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