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A photocoupler is a device incorporating a light-emitting diode (LED) and a photodetector in one package. Unlike other optical devices, light is not emitted outside the package. The external appearance is similar to that of non-isolators/solid state relays. Although a photocoupler is an optical device, it does not handle light, but handles electrical signals.
(1)The LED turns on (0 ⇒ 1).
(2)The LED light enters the phototransistor.
(3)The phototransistor turns on.
(4)Output voltage changes 0 ⇒ 1.
(1)The LED turns off (1 ⇒ 0).
(2)The LED stops light emission to the phototransistor.
(3)The phototransistor turns off.
(4)Output voltage changes 1 ⇒ 0.
*The cutaway image on the right shows a transistor-output photocoupler of the transmissive type in a double-mold structure.