Photocoupler

A signal transmission circuit composed of a light-emitting element on the input side and a light-receiving element on the output side. When an input signal is applied to the light-emitting element, it emits light and sends it to the light-receiving element. The light-receiving element detects the light and outputs an output signal. Since the transmission of on/off signals between the input and output sides is done via light, the primary and secondary sides can be completely isolated and physically separated. This configuration is often used to protect the secondary side when high voltage is present on the primary side, and it is also sometimes used as a noise filter.