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 - http://www.avagotech.com/pages/en/optical_sensors/ambient_light_photo_sensors/light_sensor_photo_sensor/. 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. Thanks, Nate
_______________________________________________ Devel mailing list Devel@lists.laptop.org http://lists.laptop.org/listinfo/devel