It takes nearly four minutes for a small flame to turn in a big fire. There should be smoke alarms in every house and office so that the first whiff of smoke can be detected to save the life of people.
Smoke detectors are of mainly two types one is the ionized detector and other is photoelectric detectors. Smoke alarms sometimes use both methods and heat detectors too. The detectors get power from the 9 volt lithium battery or 120 volts from the wiring.
Ionization chamber with the source of ionizing radiation which becomes the source of alpha particles is present in the ionization detectors. This chamber has two plates at a distance of a centimeter. When the power is supplied through battery then one plate gets positively charged and other one negatively. The alpha particles released continuously by the radiation ionize the atoms of oxygen and nitrogen air in the chamber. The negative plate attracts the positively charged atoms of oxygen and nitrogen. And negative atoms of oxygen and nitrogen gets attracted towards the positively charged plate producing a small regular electric current. Now when there is smoke in the air and it enters the chamber they neutralize the ions and they are not able to reach the plate. The current does not remain continuous in the plate and hence alarm is triggered.
In these detectors smoke comes in the way of light beam and the light beam reaches the photocell which causes the alarm. Photoelectric detectors have a chamber in T-shape which gives out a beam of light across the horizontal arm of T. A photocell that is present on the bottom of the vertical arm generates current when light reaches to it. In normal conditions the beam of light passes above the T in a regular straight line and the light does not hit the photocell placed at the right angle below the beam but when there is smoke it spreads the light which also falls on the photocells and it triggers alarm.
Both of these detectors are good to sense smoke. Photoelectric detectors respond immediately to smoldering fires and ionization detectors responds immediately to flaming fires.
Ionization detectors is cheap than the other ones but it alarms even during cooking because it is too sensitive. When the battery starts loosing its power the sound of alarms also reduces. So regular check of batteries is very necessary.