The back-bias voltage is a sensitive topic. If you could guarantee periodic clearing of the integrator, I could provide up to 10V or so. Otherwise, it should probably be limited to +3.3V.
wad On May 1, 2009, at 11:59 PM, John Watlington wrote: > On May 1, 2009, at 2:28 PM, C. Scott Ananian wrote: > >> On Tue, Apr 28, 2009 at 8:38 PM, John Watlington <[email protected]> >> wrote: >> > I don't have time to take a look at this right now, >> > but we have a A/D input to dedicate to this, if it helps work >> around >> > the patent. >> > >> > We can talk to MERL if needed. I probably still know a handfull >> > of people around there. >> >> Oh, yeah, you should be able to wire the top side of the LED >> directly to the LED and measure the photovoltaic current directly; >> that's not patented: >> battery voltage >> Q1 | >> ---from EC------|< _____ to A/D >> | >> LED V >> | >> GND >> >> The only question is whether the LED can put out enough >> photovoltaic current to be reliably measured by the A/D. > > Ahh, therein lies the challenge! > >> Depends on what the input to the A/D looks like, how much >> capacitance it sees, etc. > > Thought the KB3700 (EC) A/D datasheet frustratingly doesn't list > any such exotic parameter > as input impedance, I asked ENE and they said that the input was > high impedance CMOS > (think a MOSFET gate, in the wee, wee microamps). The impedance > also does not vary > (even though the A/D is muxed). > >> An ultralow power versoin of the 339 could fix any problems there, >> but then your parts count increases. > > Sorry, no parts count increases allowed except for one LED, > resistors, capacitors (basically free), and maybe > one transistor, diode, or NMOS MOSFET (about a penny). I will > throw in a couple of EC digital outputs, and > a day of Richard's time in EC code. > >> You don't *have* to reverse-bias the LED; that just enhances >> sensitivity, but distinguishing between >> "outside on a sunny day" and "inside" doesn't exactly require >> precision; there's at least an order of >> magnitude change in illumination, maybe 2 (http://en.wikipedia.org/ >> wiki/Lux). > > I haven't read the patent, but the advantage to a digitally > controlled time integration A/D has always > been a high dynamic range by changing the time scale. I'll let the > community suggest this > circuit/algorithm. Someone practiced in the arts might suggest > measuring the integration of > the reverse leakage current over time, using a digital output to > clear the integrator, allowing > software to control the time period over which the current is > integrated to increase the range. > > In response to earlier mails on this topic: the microphone/camera > LEDs are inviolate. > ( Ivan got this one right! ) I will not allow software > interference with those LEDs. > > The power savings resulting from switching the LEDs was significant > in Gen 1 (on the order of > 50 mW in run/suspend). Even after adding this feature, it will > have to be integrated into Ohm > and the Control Panel before being useful in power savings. > > By the way, has anyone really thought about this feature ? I grok > the intent, but you have to make > sure that kids who happen to be in brightly lit rooms (glaring > fluourescents aren't uncommon) > don't loose their backlight, and wonder why ? The keyboard > lighting on my mac is a good idea, > but they only allow adjusting the amount of light output, not the > sensitivity to ambient light. > > And the Mac's light sensor is annoyingly placed where hand movement > during typing may occlude it, > something we should be able to avoid... > The light pipes for most LEDs, however, are quite large. I believe > that a second LED > could be positioned next to an output LED, under a light pipe. > Perhaps the battery LED is > the best candidate, since it is usually not lit when operating from > the battery. > > Cheers, > wad > _______________________________________________ Devel mailing list [email protected] http://lists.laptop.org/listinfo/devel
