In the normal case, the light from the light source on the left shoots straight across and misses the sensor. When smoke enters the chamber, however, the smoke particles scatter the light and some amount of light hits the sensor: .
The sensor then sets off the horn in the smoke detector. .
Photoelectric detectors are better at sensing smoky fires, such as a smoldering mattress. .
Ionization Detectors.
Ionization smoke detectors use an ionization chamber and a source of ionizing radiation to detect smoke. This type of smoke detector is more common because it is inexpensive and better at detecting smaller amounts of smoke produced by flaming fires. .
Inside an ionization detector is a small amount of Americium-241 (perhaps 1/5000th of a gram). The radioactive element Americium has a half-life of 432 years, and is a good source of alpha particles. A typical detector contains 0.9 microcurie of Americium-241. .
An ioniziation chamber is very simple. It consists of 2 plates with a voltage across them, along with a radioactive source of ionizing radiation, like this: .
The alpha particles generated by the Americium have the following property: they ionize the oxygen and nitrogen atoms of the air in the chamber. The negative electron is attracted to the plate with a positive voltage, and the positive atom is attracted to the plate with a negative voltage (opposites attract, just like with magnets). The electronics in the smoke detector sense the small amount of electrical current that these electrons and ions moving toward the plates represent. .
When smoke enters the ionization chamber it disrupts this current - the smoke particles attach to the ions and neutralize them. The smoke detector senses the drop in current between the plates and sets off the horn. .
Disadvantage/Advantage:.
A smoke detector or a fire alarm is important in homes as well as in business institutions since they can almost immediately warn the occupants of a possible fire when its alarm is set off.