The graph above shows the outdoor light level starting from just after sunrise until after sunset, on Nov. 22 and Nov. 23 2010 in Mountain View, CA. I used a "Lux Plug" sensor from Jee Labs connected to the JeeNode wireless controller, along with three AA batteries for power. This assembly sat on a windowsill looking out, and transmitting light readings to another JeeNode connected to my desktop computer.  The value plotted is the TSL2561 light sensor, Channel 0 output, 402 msec integration, low gain mode. It saturates at a reading of 65535 when the direct sun hit it starting around 1300 hours. The rapid fluctuations in early afternoon are from the sun peeking through the branches of a tree, and hiding again.  These were cloudy days, as you can guess from how rapidly the light level changes in the morning and early afternoon, after which the clouds became higher and more wispy. Local sunset is just before 1700. The spikes around 17:20 hours are the headlights of a car driving past the window, nearly the same time on both days.  The plot below is from two nearly cloudless days, so the variations are mostly due to the sun moving past buildings and trees, and the trees blowing in the wind (Nov. 24 was more windy than Nov. 25).

The thing that surprised me was how quickly, easily, and even inexpensively you can now make such measurements. I have been playing with microcontrollers for at least 15 years, but it always took a certain amount of effort to set up a system and understand the chip setting, get the compiler working, etc. Now with the extended family of Arduino compatible microcontrollers around that use a friendly software environment, it has become very easy to do such things.  I have used an Arduino 2009 and a "Boarduino" from Adafruit, but I found the Jee Labs system is especially convenient for small standalone battery-powered sensors with short-range wireless data links back to my main PC.

 Local Sunrise: 06:58  Sunset: 16:52  Noon: 11:55     from NOAA Calculator

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