Optocouplers are capable of transferring an electrical signal between two circuits while electrically isolating the circuits from each other. They generally consist of an infrared LED emitting section at the input and a silicon photodetector at the output. In Often, other circuitry is included as part of the device. The input for optocouplers can be AC or DC. The output can be a photocell, photodiode, phototransistor, or photodarlington. Photocells are light-dependent resistors. They are used to detect changes in light intensity. Photodiodes are a two-electrode, radiation-sensitive junction formed in a semiconductor material in which the reverse current varies with illumination. A phototransistor is a bipolar transistor used as a photodetector. It provides current at its output that is proportional to light intensity at its input. The low-level input light current is amplified by the current gain (beta) of the transistor. A photodarlington is a pair of bipolar transistors connected in a Darlington configuration to provide very high current gain and often used as the photodetector section of an optocoupler.