Miniature Photocell - Light Sensor

These little light sensors play well on an analog input pin, with a pullup resistor of 10K. Resistance changes from ~1KΩ when it's light to ~10KΩ in the dark, so it is easy to sense daylight, cabin lights coming on, abrupt shadows on solar panels, access panels opening, passing flashlight beams, and other events that signal themselves with photons. A must in any boat-hacker's sensor toolkit...