The CT357B(T1) is an optocoupler, a kind of small component that contains one or more light-emitting diodes (LEDs) and a photosensitive transistor. It is manufactured by the company CT Micro International, and is a SOP-4_P2.54 RoHS compliant model. It is designed to isolate one electrical circuit from another, and works by having a photonic signal travel between the LED and the phototransistor, which can be used to control or supply a current to the second circuit. This model has an encapsulated plastic body, a 4-pin SOP package, and a pitch of 2.54mm. It provides high isolation voltage ratings of 5kV (rms) and a current transfer ratio range of 100% to 400% when driven with 30mA. It is widely used in industrial automation systems, telecommunications, entertainment systems, and more.