Optocouplers (also called an opto-isolators , photocouplers, or optical isolators) is an electronic component that transfers electrical signals between two isolated circuits by using light.^[1] Opto-isolators prevent high voltages from affecting the system receiving the signal.^[2] Commercially available opto-isolators withstand input-to-output voltages up to 10 kV^[3] and voltage transients with speeds up to 25 kV/μs.^[4]

A common type of opto-isolator consists of an LED and a phototransistor in the same opaque package. Other types of source-sensor combinations include LED-photodiode, LED-LASCR, and lamp–photoresistor pairs. Usually opto-isolators transfer digital (on-off) signals and can act as an electronic switch, but some techniques allow them to be used with analog signals.