The most widely used smoke detectors are photoelectric and ionization smoke detectors
Photoelectric smoke detector
This works on the principle of light scattering principle.the smoke sensing chamber contains an IR led source with a peak spectral emission of 880 nm .This source is placed at an angle of from a spectrally matched photo diode receiver During a no smoke condition,only light reflected from the chamber walls enter the receiver and shows up as a small photo current.
As the smoke particles enter the sensing chamber and cross the light beam of LED more light reaches the receiver due to scattering.The receiver circuitry converts this photo current into a signal voltage.In a detector when this voltage reaches a preset level an alarm is produced.In a sensor this signal voltage goes into an 8 bit A to D converter A digital representation of this signal voltage is then transmitted to the fire alarm panel for further processing
Ionization Smoke detector
This device use a small radiation source, Americium 241 which emits alpha particles that ionize air molecules between two electric allies charged electrodes.With the application of a DC volt. to these electrodes, a small ionization current flows within the chamber .
As the smoke enters the chamber a decrease in ionization current results .This current is converted into a signal voltage by a trans impedance circuit.In a detector when this signal voltage drops below a preset level an alarm is produced In a sensor this signal voltage goes into a 8 bit A/D C .A digital representation of this signal voltage is then transmitter to FAP for further processing
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