That wonderful stream of photons that drives photosynthesis inside the chloroplasts. Our little $0.95 photocells preformed great in their first test. In the dark of a closet they measured 500,000 Ohms and outside in the mid afternoon sun they measured 50 Ohms, a 4 log range! Very exciting.
It is reading from about 550nm to 650nm light (green-yellow range) which is what our eyes see the best. The plants however will prefer more blue (400nm-550nm) and red (650nm-700nm) which is hopefully what the LED lights will provide. But these sensors do a good job of reporting the relative intensity of the light and thats all we need from them!
In the future they will be used to monitor light levels inside the greenhouse chamber and potentially turn on the supplemental LED lights on cloudy or rainy days.
Closet: 500,000 ohms
Indoors, natural light: 1000 ohms
Indoors lights on: 1000-400 ohms
In front of an LED flashlight: 300 ohms
Outside in the midday sun: 50 ohms
It is however, an analog device. So we need an ADC to convert the signal so that the digital Pi can read it. Or we upgrade to the most expensive digital lux sensor that is more accurate and precise but about 10x more expensive. It also has a much wider range of light it can see; from visible all the way to IR.
Cheap analog sensor:
http://www.adafruit.com/products/161#Description
More expensive digital sensor:
http://www.adafruit.com/products/439
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