Smoke detector
Fire safety systems in buildings typically include networks of devices that detect fire conditions, devices that provide notification of fire conditions, and devices that perform specialized control operations during the existence of fire conditions. Smoke detectors have been recognized as being very useful and desirable in providing an early warning of dangerous levels of smoke, usually due to fire. When such detectors go into alarm, they usually generate an audible alarm indicating output. Smoke detectors are used in closed structures such as houses, factories, offices, shops, ships, aircraft, and the like, with virtually every building being required by law to have some sort of early warning fire detection system. Smoke detectors operate on the principle of detecting the presence of a certain level of smoke particles within the area being monitored. Once the threshold level of smoke particles in the area has been exceeded, the smoke detector indicates the alarm condition. Such smoke detectors may operate on photoelectric light scattering principle, or on an ionization principle. The working principle of the photoelectric smoke detectors is that the combustion particles entering the detector's chamber cause the infrared light (IR) emitted from an IR LED to be scattered so as to be detected by the photodiode. Typically, a light emitting diode (LED) transmits light into a detection chamber where it is absorbed by a labyrinth structure. When smoke, due to a fire, is present in the air entering the detection chamber, the smoke particles scatter the light generated by the LED. The intensity of the light sensing by an IR photodiode receiver will be transformed into an electrical signal and sent to the fire alarm receiver or the fire alarm siren by the interconnected electrical circuits. The smoke sensing chamber is generally designed to exclude most ambient light influences while providing openings to permit entry of the smoke particles into the sensing chamber. Ionization smoke detectors depend on detecting adsorption of smoke particles on atmospheric ions, and the latter on optical effects in aerosols, e.g., the extinction of a beam of light by smoke or the scattering of light by smoke particles. Smoke detectors are often battery powered, but may also be supplied from the wiring of a house. Where supplied from house wiring, a battery backup is still used to ensure proper operation of the smoke detector in the event a fire interrupts power to the smoke detector.
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