An optocoupler is an electronic device that allows to transmit an electrical signal between two isolated circuits. It does this by using an infrared LED and a photosensitive device that detects said light. When a current is applied to the optocoupler, the LED emits an infrared light that is detected by the photosensitive device. This causes the photosensitive device to start conducting current powering the second device.

Optocouplers