Re: Ambient light sensing via LED response
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 http://lists.laptop.org/listinfo/devel
[Fwd: Re: Ambient light sensing via LED response]
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
Re: [Fwd: Re: Ambient light sensing via LED response]
C. Scott Ananian wrote: On Wed, May 6, 2009 at 6:14 PM, Reinder de Haan r...@mveas.com wrote: 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. I think the actual A/D reference voltage is probably *maximum* 3.3V. What's the *minimum* A/D reference voltage? And can the A/D measure all the way down to ground? (Sometimes there's a Vmin for the A/D input, often around a diode drop above ground.) 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...) that 250mv is without the light guide and lcd cover installed! with the light guide in place its more like ~70mv ... 0.07/3.3*256= 5.4 LSB and then the Rin of the ADC must be 10Mohm... 250mv should be a count of about 20 from the A/D. That's plenty for this purpose. Heck, a count of 1 would be sufficient, as long as it was repeatable. ;-) So, it seems like all that's required is one wire from the top of the LED to the A/D input, and wiring Vref directly to +3.3. Adding a zener and a resistor for a lower Vref would probably improve precision and accuracy. Everything else is software tweaking some constants. --scott if i find the time somewhere this week i might test it ... otherwise it will be during of after paris.. Reinder ___ Devel mailing list Devel@lists.laptop.org http://lists.laptop.org/listinfo/devel
Re: Longer XO transformer power cord in the plans?
Sascha Silbe wrote: On Sun, May 31, 2009 at 03:16:53PM +1000, James Cameron wrote: 1. Earthing. The current design has no earth at the AC end, and is isolated in relation to the DC end. An earthed AC plug in some countries produces a more reliable and positive insertion and anchoring. All travel adapters (power outlet adapters) I've come across so far had no earthing so would be impossible to use (unaltered). Of course this wouldn't be much of a change as the current wall warts also don't fit any adapter I've seen at shops. At SugarCamp in Paris, quite a few people (including myself) had custom ones, i.e. with mechanic alterations. Personally, I feel comfortable making minor mechanic changes to an adapter, but I won't usually dare using a non-earthing adapter with a device having an earthed plug (unless I know for sure this is safe). Actually, if you are able to use a standard plug (e.g. IEC-60320-C5/C6) at the power supply end, above won't apply at all as it's usually easy to get a matching cable, no travel adapter needed. :) +1 for inline adaptor its MUCH easier to exchange only the mains cable: 1) when its demaged 2) when shiping to a different part of the world you would need only one powersupply brick for (almost?) all or the world. i have seen some companies ship a couple of different cables so the device is usable almost everywhere and doesn't need to be custom packed/country. i would go which IEC-60320 C8/C9 which is used for half of the laptops today. i feel a earthed design only increases the risks, even more so when you cant depend on the quality of the mains supply. the only advantage to the earthed design that im aware of is that the power supply easier(cheaper?) meets EMC/FCC regulations. I hope future XO versions will still have the same broad power input specs as the XO-1. It's been very useful already (e.g. cable-only car adapter, no voltage conversion or even voltage limit necessary). CU Sascha ___ Devel mailing list Devel@lists.laptop.org http://lists.laptop.org/listinfo/devel ___ Devel mailing list Devel@lists.laptop.org http://lists.laptop.org/listinfo/devel
Re: Longer XO transformer power cord in the plans?
Tiago Marques wrote: On 5/31/09, Reinder de Haan r...@mveas.com wrote: Sascha Silbe wrote: On Sun, May 31, 2009 at 03:16:53PM +1000, James Cameron wrote: 1. Earthing. The current design has no earth at the AC end, and is isolated in relation to the DC end. An earthed AC plug in some countries produces a more reliable and positive insertion and anchoring. All travel adapters (power outlet adapters) I've come across so far had no earthing so would be impossible to use (unaltered). Of course this wouldn't be much of a change as the current wall warts also don't fit any adapter I've seen at shops. At SugarCamp in Paris, quite a few people (including myself) had custom ones, i.e. with mechanic alterations. Personally, I feel comfortable making minor mechanic changes to an adapter, but I won't usually dare using a non-earthing adapter with a device having an earthed plug (unless I know for sure this is safe). Actually, if you are able to use a standard plug (e.g. IEC-60320-C5/C6) at the power supply end, above won't apply at all as it's usually easy to get a matching cable, no travel adapter needed. :) +1 for inline adaptor its MUCH easier to exchange only the mains cable: 1) when its demaged 2) when shiping to a different part of the world you would need only one powersupply brick for (almost?) all or the world. i have seen some companies ship a couple of different cables so the device is usable almost everywhere and doesn't need to be custom packed/country. i would go which IEC-60320 C8/C9 which is used for half of the laptops today. Completely not the picture around this part of Europe. Most of them come with C13 and some are being sold with C7, which is pretty much a oops i meant C7/C8 NOT C9!! standard for other types electronics. C13 would be my favorite, if the size of the plug is not an issue, since it is also the standard for computer power supplies. As mentioned above, C5 would also be sweet. both are an earthed connector and so 'require' an earthed outlet. combined with that almost all power supplies i have seen with a C5/C6 or C13/C14 connect the earth input to the ground/0V dc output... if you insert such power supply into an not earthed outlet (which are quite common) your whole laptop will be at ~1/2*Uin Vac due to the filter capacitors in the mains filter. which gives a nasty shock if you touch both a non insulated part of your laptop and a earthed object.. As for safety, adding something like the cable plug of the original Xbox pads would work perfectly and it's not as expensive as Apple's magnetic plug. Cost of this is something I have no clue about. Best regards, Tiago Marques i feel a earthed design only increases the risks, even more so when you cant depend on the quality of the mains supply. the only advantage to the earthed design that im aware of is that the power supply easier(cheaper?) meets EMC/FCC regulations. I hope future XO versions will still have the same broad power input specs as the XO-1. It's been very useful already (e.g. cable-only car adapter, no voltage conversion or even voltage limit necessary). CU Sascha ___ Devel mailing list Devel@lists.laptop.org http://lists.laptop.org/listinfo/devel ___ Devel mailing list Devel@lists.laptop.org http://lists.laptop.org/listinfo/devel ___ Devel mailing list Devel@lists.laptop.org http://lists.laptop.org/listinfo/devel
