Photocells are made from a compound called cadmium sulfide that changes in resistance when exposed to varying degrees of light. Cadmium sulfide photocells are most sensitive to visible red light, with some sensitivity to other wavelengths. Photocells have a relatively slow response to changes in light. The characteristic blinking of overhead ourescent lamps, which turn on and 0 at the 60 Hertz line frequency, is not detected by photocells. This is in contrast to photo transistors, which have frequency responses easily reaching above 10,000 Hertz and more.
Applications
Photocells can be used to detect the incandescent lamp that acts as a contest start indicator. They also can be used to measure the reactivity of the playing old surface if coupled with a light source such as a red LED or an incandescent lamp.
Photocells can be used to detect room ambient lighting. With the use of polarizing liters placed immediately in front of the photocell element, an array of such photocells can be used to detect the polarization angle of polarized light present in ambient lighting. Robot-level control programs can be used to seek out such sources of polarized light.
Electrical Data
Photocells yield a decrease in resistance with an increase in illumination. Because of the wiring of the voltage divider (the photocell is on the lower rung), an increase in light will correspond to a decrease in voltage measurement.
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