C. Scott Ananian wrote: > On Wed, May 6, 2009 at 6:14 PM, Reinder de Haan <[email protected]> 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 [email protected] http://lists.laptop.org/listinfo/devel
