-------- Original Message -------- Subject: Re: Ambient light sensing via LED response Date: Thu, 07 May 2009 00:02:49 +0200 From: Reinder de Haan <r...@mveas.com> To: raf...@laptop.org References: <200904290017.n3t0hl2v006...@new.toad.com> <c5d4a4e5-b183-4b7d-bafb-3488e15f7...@laptop.org> <c6d9bea0905011128j2d5cdf3cw975cfad790480...@mail.gmail.com> <675d3c5c-95ed-4ee9-84bc-3b5164675...@laptop.org> <c6d9bea0905042207i458ed9b1if74e0e16d24f5...@mail.gmail.com> <4a00a073.8020...@mveas.com> <a80d16920905051544s525b7039nf7e87a4a3c754...@mail.gmail.com> <75dc0458-925b-4792-9aee-a24b9b1f3...@laptop.org> <a80d16920905051952h3d82d15brb8f690cf1c402...@mail.gmail.com>
Rafael Enrique Ortiz Guerrero wrote: > > > > On Tue, May 5, 2009 at 8:34 PM, John Watlington <w...@laptop.org > <mailto:w...@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 <r...@mveas.com > <mailto:r...@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 > > > > > > > > > _______________________________________________ > Devel mailing list > Devel@lists.laptop.org <mailto:Devel@lists.laptop.org> > http://lists.laptop.org/listinfo/devel > > _______________________________________________ > Devel mailing list > Devel@lists.laptop.org <mailto:Devel@lists.laptop.org> > http://lists.laptop.org/listinfo/devel > > > _______________________________________________ Devel mailing list Devel@lists.laptop.org http://lists.laptop.org/listinfo/devel