Smoke Alarms Saves Live
How Do Smoke Detectors Work?
Photoelectric & Ionization Smoke Detectors
Here are two main types of smoke detectors: ionization detectors
and photoelectric detectors. A smoke alarm uses one or both methods,
sometimes plus a heat detector, to warn of a fire.
Types of Smoke Detectors
Ionization detectors have an ionization chamber and a source of ionizing
radiation. The source of ionizing radiation is a minute quantity of americium
241 (perhaps 1/5000th of a gram), which is a source of alpha particles
(helium nuclei). The ionization chamber consists of two plates separated by
about a centimetre. The battery applies a voltage to the plates, charging one
plate positive and the other plate negative. Alpha particles constantly
released by the americium knock electrons off of the atoms in the air,
ionizing the oxygen and nitrogen atoms in the chamber. The positively
charged oxygen and nitrogen atoms are attracted to the negative plate and
the electrons are attracted to the positive plate, generating a small,
continuous electric current. When smoke enters the ionization chamber,
the smoke particles attach to the ions and neutralize them, so they do not
reach the plate. The drop in current between the plates triggers the alarm.
In one type of photoelectric device, smoke can block a light beam. In this case, the reduction in light reaching a photocell sets off the alarm. In the most common type of photoelectric unit, however, light is scattered by smoke particles onto a photocell, initiating an alarm. In this type of detector there is a T-shaped chamber with a light-emitting diode (LED) that shoots a beam of light across the horizontal bar of the T. A photocell, positioned at the bottom of the vertical base of the T, generates a current when it is exposed to light. Under smoke-free conditions, the light beam crosses the top of the T in an uninterrupted straight line, not striking the photocell positioned at a right angle below the beam. When smoke is present, the light is scattered by smoke particles, and some of the light is directed down the vertical part of the T to strike the photocell. When sufficient light hits the cell, the current triggers the alarm.