Optocoupler

An optocoupler passes a signal using light, keeping two circuits electrically separate. Inside, an infrared LED controls a phototransistor. The infrared light is invisible to our eyes.

Each side needs its own power. Use a resistor for the input LED and keep the grounds separate to preserve isolation.

When the last sack of flour is ready at SparkVille’s mill, an optocoupler carries the news to the bakery’s bell controller.