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Broadly speaking, an optocoupler is a device which transmits a signal via light, requiring no electrical connection between the transmitter and receiver. Usually when speaking about an optocoupler component, we mean a device (say, a 4- or 6-pin IC) which has an input on one side and an output on the other side. Inside the device is a tiny light source and a tiny light detector which are not connected electrically. By applying an input to one side, you can make the tiny light (typically an LED) light up, and then read the result from the other side (which is connected to the tiny light detector).