With an optocoupler you transmit signals. A signal is not transmitted electrically, but by light from the LED to the phototransistor. Hence there is no electrical connection between the LED and the phototransistor.
And this has advantages, because now you can transmit signals back and forth between two electrically completely separated circuits. Therefore, you can find optocoupler in, for example, control of heavy machinery, medical equipment and computers. Optocouplers are available in different designs, depending on the specific application and the number of signals to be transmitted.
On the following pages you get to know optocouplers and their applications.
To get started you need to do a few preparations...