Smoke detector

A smoke detector is a safety device that detects airborne smoke and issues an audible alarm, thereby alerting nearby people to the danger of fire. Smoke detectors work either by optical detection or by ionization.

Optical detector

This type of detector includes a light source, a lens to collimate the light into a beam, and a photodiode or other photoelectric sensor at right-angles to the beam. In the absence of smoke, the light passes in front of the detector but does not fall on it. When visible smoke enters the beam, some light is scattered by the smoke particles, and some of the scattered light is detected by the sensor. An increased output from the sensor sets off the alarm.

Ionization detector

This type of detector is cheaper than the optical detector, and can detect particles of smoke that are too small to be visible. It includes a tiny mass of radioactive americium-241, which is a source of alpha radiation. The radiation passes through an ionization chamber, which is an air-filled space between two electrodes, and causes a small, constant current to flow between the electrodes. Any smoke that enters the chamber interrupts this flow of current, setting off the alarm.