On Fri, May 1, 2009 at 2:28 PM, C. Scott Ananian <csc...@laptop.org> wrote:

> 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).

A reverse biased LED doesn't output much current at all - even photo sensor
diodes that are tuned for the job. Have a look at this page for some ambient
light sensors that you would find in a cell phone -
The basic variety has a photodiode and a small current amplifier. The output
current is logarithmically related to the lux level. If hook a series
resistor to the output, the voltage across vs lux is also logarithmic.
Besides the current amplifier, these devices have other advantages. First,
they have a spectral sensitivity that's tuned to the human eye, so infrared
light (say a campfire) will not skew the reading. Second, they have a large
optical window so the readings won't change when you tilt your phone/laptop
slightly. These little sensors are less than $0.25 in large quantities
(perhaps by quite a bit), and they are being used more and more in cell
phones to reduce backlight power in low light environments. It's well worth
the price if you get 10% more battery life on average. Quantifying the
impact on battery life is tricky because there are a lot of assumptions.

Anyways, a normal LED might work under certain circumstances. I don't have
the experience to say one way or another.

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