An optocoupler, also known as an opto-isolator, photocoupler, or optical isolator, is an electronic component that transfers electrical signals between two isolated circuits by using light. It consists of a light emitter, usually a near-infrared LED, and a light-sensitive receiver, which can be a single photodiode, phototransistor, or other photosensitive device. The emitter and receiver are separated by a closed optical channel, also called a dielectric channel, which prevents high voltages from affecting the system receiving the signal. Optocouplers can be used to remove electrical noise from signals, isolate low-voltage devices from high-voltage circuits, and allow small digital signals to control larger AC voltages. They can also be used as a switching device or with other electronic devices to provide isolation between low and high-voltage circuits. Optocouplers come in four configurations, each sharing the same infrared LED with a different photosensitive device: photo-transistor and photo-darlington, which are typically used in DC circuits, and photo-SCR and photo-TRIAC, which are used to control AC circuits. Optocouplers can be used in various applications, such as microprocessor input/output switching, DC and AC power control, communications equipment protection, and power supply regulation. The choice of optocoupler depends on the intended application, and the most important parameter is the Current Transfer Ratio (CTR), which represents the ratio between the output and input currents.