How does a smoke detector work?


Quick Answer

The two most common types of smoke detectors are ionization detectors and photoelectric detectors. Ionization smoke detectors sound when smoke blocks the flow of ions they generate. A photoelectric smoke detector goes off when smoke particles bounce light from its LED against its photocell.

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Full Answer

Ionization detectors are sophisticated devices that use ionizing radiation to detect the presence of smoke. Their ionization chambers are constructed with two plates and a small amount of the radioactive isotope Americium-241. The battery of the smoke alarm charges the plates, one positively and the other negatively, and as the Americium-241 decays at a constant rate, particles coming into the chamber ionize. Because ions are attracted to the opposite electrical charge, they generate a small flow of electricity between the plates. When enough smoke interrupts the electrical current, the smoke detector goes off.

Simpler devices, photoelectric, or optical, smoke detectors are constructed with T-shaped chambers that have an LED at the top of the T and a photocell at the bottom. When smoke particles enter the T, they interrupt and redirect the beam of light emitted by the LED. When enough redirected light strikes the photocell at the other end of the chamber, the alarm goes off.

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