A photocoupler is a component that allows the transfer of signals between two circuits using light radiation as the transmission medium. The two systems are galvanically isolated from each other, both security of the entire system and the operator. Are formed by a sender, which in most cases is a LED and a photodetector that typically identifies phototriac or phototransistor.
The PC817B has a NPN transistor output and an infrared light emission source of gallium arsenide LED. It is largely used in switching power supplies and stabilizer, in robotics and automatic control systems.
Isolation voltage: 5KV
CTR: 130 ~ 260%
Forward voltage: 1.2 ~ 1.4 VDC
Maximum forward current: 50mA
Collector current: 50mA (max.)
Power dissipation: 150mW (max.)
For more info download datasheet.