Transistor output optocouplers are electronic components that allow a current to flow between two isolated circuits by using a light-emitting diode (LED) and a phototransistor. The LED and phototransistor are optically coupled, meaning that the current can flow from one circuit to the other without a direct electrical connection. The output transistor can provide current gain or the ability to control the voltage levels. These components are used to isolate the circuit from external interferences, provide electrical isolation and filtering, and prevent ground loops. They also provide electrical noise and transient protection, allowing signals to travel with minimal distortion. Transistor output optocouplers are commonly used in applications such as motor control, instruments, and sensors.