PC817 (also known as EL817) is a phototransistor optocoupler with a low input current and a high gain. It is made up of a phototransistor that is optically connected to a gallium arsenide infrared-emitting diode and is placed in a compact 4-pin device.

The PC817 is extensively used in electronic circuits for electrical isolation, signal amplification, and voltage level shifting. It offers electrical isolation between two circuits, enabling signal transmission without any physical connection, which is particularly beneficial in situations requiring high voltage or current separation.

The PC817 is appropriate for use in a broad range of industrial and consumer electronics applications due to its wide working temperature range of -55°C to 110°C and high isolation voltage of 5000Vrms. It is also simple to operate and needs just a few external components, making it an affordable option for a wide range of applications.