How optical smoke detectors work

The answer to that question is really two answers, because there are two quite different kinds of smoke detectors. One is a kind of electronic eye; the other's a sort of electronic nose. The eye type of detector is more properly called an optical smoke detector (or photocell smoke detector) and it works a bit like Tom Cruise in Mission Impossible. Remember the scene when Tom dangles from the ceiling trying to avoid all those light-detecting burglar beams? An optical smoke detector is just like that inside.

The detector must be screwed to your ceiling because that's where smoke heads for when something starts to burn. Fire generates hot gases and because these are less dense (thinner or weigh less per unit of volume) than ordinary air they rise upward, swirling tiny smoke particles up too. The detector is designed with a large opening in the bottom (1), shown upper right in our top photo, that leads to the detection chamber up above. An invisible, infrared light beam, similar to the ones that Tom Cruise dodged, shoots across the chamber from alight-emitting diode or LED (2) to a photocell (3). The photocell is an electronic light detector that generates electricity for as long as light falls on it. Normally, when there is no smoke about, the light beam shoots constantly between the LED and the detector. An electronic circuit (4) detects that all is well and nothing happens. The alarm (5) remains silent.

But if a fire breaks out, smoke enters the chamber (6) and interrupts the beam (7). Because no light is falling on the photocell, it does not generate an electric current anymore. The circuit spots this straight away (8), realizes something's amiss, and triggers the shrill and nasty alarm (9) that wakes you up and saves your life.

Ionization Smoke Detector

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