Which device is known for isolating circuits while allowing light to pass?

The optocoupler is a device specifically designed to isolate different parts of a circuit while allowing light to pass through. It typically consists of a light-emitting diode (LED) and a photodetector, such as a phototransistor, housed together. When current flows through the LED, it emits light, which then activates the photodetector. This configuration ensures electrical isolation between the input and output circuits, allowing signals to be passed without a direct electrical connection. This isolation is crucial in many applications where it is necessary to protect sensitive components from high voltages or transient spikes.

In contrast, transistors and photodiodes do not provide isolation in the same manner as optocouplers. A transistor is used primarily for amplification or switching but does not inherently provide isolation through light. Photodiodes convert light into electrical signals but do not isolate circuits; instead, they require a direct electrical connection. Transformers, while capable of isolating circuits through electromagnetic induction, do not allow light to pass and operate primarily with alternating current (AC) rather than light. Hence, the optocoupler is the only device that effectively combines circuit isolation with the capacity for optical signal transmission.