How Does a Smoke Detector Work?

Ionisation smoke detectors contain a tiny amount of americium-241, a radioactive isotope that ionises air molecules between two electrically charged plates, creating a small steady current. When smoke particles enter the chamber, they attach to ions and reduce the current, triggering the alarm. Photoelectric (optical) detectors shine an LED light beam into a sensing chamber; when smoke particles scatter the light onto a photosensor, the alarm sounds. Dual-sensor models combine both technologies. Ionisation types respond faster to flaming fires, while photoelectric types detect smouldering fires more quickly. In many countries, regulations now favour photoelectric or dual-sensor units for residential use.