What is the most common configuration of an optoisolator or optocoupler?

An optoisolator, or optocoupler, is an electronic component that transfers electrical signals between two isolated circuits using light. This optical link prevents high voltages or transient voltage spikes on one side of the circuit from damaging the receiving components on the other side. The most common and practical internal configuration consists of a Light Emitting Diode (LED) paired with a phototransistor. The LED converts the input electrical signal into a light beam, and the phototransistor detects that light to recreate the electrical signal. Distractors mentioning lasers or photomultipliers describe highly complex, specialized equipment, not standard circuit board isolators.