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

Mark Bauer mark at zjunk.net
Sun May 10 19:02:55 EDT 2009

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Hi guys,

How can I help with this,  I still teach electrical engineering at the
University of Nebraska, and before that I have about 25 years in
the field.  Mostly embedded microcontrollers and stuff.  I do have
several XOs to work with.

I would have jumped in earlier but the end of the semester is a bit
hectic.

Mark

On May 10, 2009  Sunday, at 4:13:47:0, C. Scott Ananian wrote:

> On Sun, May 10, 2009 at 5:01 PM, Reinder de Haan <rs at mveas.com> wrote:
>> 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...
>
> 5 LSB is still okay, and it could be more if the A/D Vref can be
> dropped below 3.3V.
> --scott
>
> --
>                         ( http://cscott.net/ )
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