![]() ![]() Like photodiodes, phototransistors may have filters to alter their sensitivity spectrum and lenses to control their viewing angle. For a sufficiently large resistance R, the sensor’s output voltage ranges from close to 0 V (no light on the phototransistor) to close to 3.15 V (transistor saturated, with 0.15 V drop from collector to emitter). The resistance R should be chosen to get the right voltage range at the input to the PIC32, which could be an analog or digital input, depending on the application. (Right) A circuit with a WP7113SRC/DU red LED illuminating an SFH 310 phototransistor. (Image courtesy of Digi-Key Electronics, .) (Middle) The circuit symbol for an NPN phototransistor. The shorter leg is the collector and the longer leg is the emitter.
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