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Optoisolator Pinout.svg

In electronics, an opto-isolator, also called an optocoupler, photocoupler, or optical isolator, is a component that transfers electrical signals between two isolated circuits by using light. Opto-isolators prevent high voltages from affecting the system receiving the signal. Commercially available opto-isolators withstand input-to-output voltages up to 10 kV and voltage transients with speeds up to 10 kV/μs.

A common type of opto-isolator consists of an LED and a phototransistor in the same opaque package. Usually opto-isolators transfer digital (on-off) signals, but some techniques allow them to be used with analog signals.


An opto-isolator connects input and output sides with a beam of light modulated by input current. It transforms useful input signal into light, sends it across the dielectric channel, captures light on the output side and transforms it back into electric signal. Unlike transformers, which can pass energy in both directions with low losses, opto-isolators are unidirectional and they cannot transmit power. Typical opto-isolators can only modulate the flow of energy already present on the output side. Unlike transformers, opto-isolators can pass DC or slow-moving signals and do not require matching impedances between input and output sides. Both transformers and opto-isolators are effective in breaking ground loops, common in industrial and stage equipment, caused by high or noisy return currents in ground wires.

DCC Applications

Boosters with opto-isolated inputs are available, a typical application would be use with a layout with common rail wiring. This prevents damage from incorrect signals being applied to your booster's input. This is why common rail wiring is discouraged with DCC.