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A smoke detector or smoke alarm is an active fire protection device that activates an audible alarm on detection of airborne smoke, thereby alerting nearby people on event of any fire breakage.
Working
The working of smoke detector is based on either optical detection method or on ionization method. However, there are some detectors which use both detection methods to increase sensitivity to smoke. Smoke detectors may be powered by a 9-volt battery, lithium battery, or 120-volt house wiring.
Types of detectors
Ionization Detectors
Ionization detectors consist of an ionization chamber and a source of ionizing radiation.
Source of ionizing radiation
The source of ionizing radiation usually contains americium-241 (perhaps 1/5000th of a gram)-- a source for generating alpha particles (helium nuclei). These alpha particles are capable of ionizing the oxygen and nitrogen atoms of the air in the chamber.
Ionization chamber
The ionization chamber comprises of two plates separated by a distance of one centimeter. The ionization chamber consists of two plates with a voltage across them, with a radioactive source of ionizing radiation. By doing so, radiation is produced which passes through an air-filled space between two electrodes, and permits a small, constant current to flow between the electrodes.
Working of ionization detectors
The americium releases alpha particles at a constant rate, which knocks electrons off of the atoms in the air, thus, ionizing the oxygen and nitrogen atoms in the chamber. Due to this, the positively charged oxygen and nitrogen atoms are attracted towards the negative plate and the electrons are attracted towards the positive plate, generating a small, continuous electric current. When smoke enters the ionization chamber, it absorbs the alpha particles leading to reduction of the ionization process and flow of current. Consequently, the change in the ionization process activates off the alarm.
Photoelectric Detectors
In this type of detectors, alarm is set off due to interruption of path of light by smoke.
Structure
A Photoelectric detectors comprises of T-shaped chamber with a light source (usually LED) and a sensor, but they are positioned at 90-degree angles to each other. A photocell, positioned at the bottom of the vertical base of the T, generates a current when it is exposed to light
Working
In the normal condition (i.e. when smoke is not present), the light emitted from the light source travels in an uninterrupted straight line perpendicular to the sensor. However the moment smoke enters the chamber, the light gets scattered by the light due to which some light gets directed towards sensor. When sufficient light hits the cell, current activates the sensor, which in turn sets off the horn in the smoke detector.
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