what is the working principle of smoke detector in fire
alarm system
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Answer Posted / priya subha
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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