Working of Smoke Detectors

WORKING OF SMOKE DETECTORS.

 
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)

Smoke  Detector

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. As you can see in the photo up above, the detector has slits around its case (1), which lead to the main detection chamber. An invisible, infrared lightbeam, similar to the ones that Tom Cruise dodged, shoots into the chamber from a light-emitting diode (LED)(2). The same chamber contains a photocell (3), which is an electronic light detector that generates electricitywhen light falls on it. Normally, when there is no smoke about, the light beam from the LED does not reach the detector. An electronic circuit (4), monitoring the photocell, detects that all is well and nothing happens. The alarm (5) remains silent.
But if a fire breaks out, smoke enters the chamber and scatters some of the light beam into the photocell . This triggers the circuit  setting off the shrill and nasty alarm (9) that wakes you up and saves your life.

How ionization smoke detectors work

Ionization Detector

Another type of smoke alarm is less expensive than the optical type, more common, and works in a totally different way. You can think of it as an electronic nose because, like the nose on the front of your face, it uses a kind of chemistry to spot unusual molecules (smoke) heading inward. Detectors like this are called ionization smoke detectors.
 Inside the detector, there's an ionization chamber open to the air (1) filled with ions (2), which, in this case, are atoms that have lost electrons to make positively charged nuclei. Where do the ions come from? Inside the chamber, there's a small piece of a chemical element called americium (3). It constantly spews out tiny radioactive particles (called alpha particles), which leak into the detection chamber. As they do so, they crash into air molecules and turn them into positively charged ions (shown here as big red blobs) and negatively charged electrons (shown as smaller black blobs). The ions and electrons whiz in opposite directions between two electrodes (electrical contacts, rather like the terminals of a battery). As long as the ions and electrons are moving, a current flows between the electrodes and a circuit (4) in the smoke detector thinks all's well, so the alarm (5) remains silent.
However, if a fire breaks out, smoke particles get into the detector and start to clog up the ionization chamber (6). They attach themselves to the ions and effectively shut off the electric current (7). The circuit in the detector spots that change straight away (8) and sounds the alarm (9). Once the fire is out and the smoke is gone, the detection chamber clears, the ions travel back and forth between the electrodes as before, the circuit shuts down, and the alarm stops sounding.
You can see an example of an ionization detector, opened up, in the photo below:

Which is best, ionization or photoelectric?
According to a detailed study by the US National Institute of Standards and Technology: "Ionization type alarms provided somewhat better response to flaming fires than photoelectric alarms, and photoelectric alarms provide (often) considerably faster response to smoldering fires than ionization type alarms." Smoke detectors are trivially inexpensive compared to the cost of fire damage—and life, of course, is priceless. Although any smoke alarm is better than none, fit both types of detector if you possibly can.