An optocoupler

An optocoupler, also called an optoisolator, transfers electrical signals between two circuits using light while keeping them electrically isolated. The PC817 contains an infrared LED at pins 1 and 2 and a phototransistor at pins 3 and 4. When the digital input is 0 V, the LED remains OFF, so the phototransistor does not conduct. The pull-up resistor R2 raises the output to 12 V. When the input becomes 5 V, current flows through R1 and the LED emits light. This light turns ON the phototransistor, allowing current to flow from collector to emitter and pulling the output down to 0 V. Because only light crosses the isolation barrier, the low-voltage control circuit remains protected from noise, voltage spikes, and faults present in the 12 V output circuit.