[Fwd: Re: Ambient light sensing via LED response]

[Reinder de Haan]

C. Scott Ananian wrote:
> On Wed, May 6, 2009 at 6:14 PM, Reinder de Haan <rs at 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