Ambient light sensing via LED response

[C. Scott Ananian]

On Tue, Apr 28, 2009 at 8:38 PM, John Watlington <wad at laptop.org> 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.  Depends on what the input to the A/D
looks like, how much capacitance it sees, etc.  An ultralow power versoin of
the 339 could fix any problems there, but then your parts count increases.
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).
 --scott

-- 
                        ( http://cscott.net/ )
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