Photoelectric Smoke Detectors/Sensors; Ionization Smoke Detectors/Sensors - Simplex 4098 Applications Manual

PRINCIPLES OF OPERATION, Continued
Photoelectric Smoke
Detectors/Sensors
Ionization Smoke
Detectors/Sensors
These devices operate on a light scattering principle. The smoke sensing
chamber contains an infrared LED source with a peak spectral emission of 880
nanometers. This source is placed at an angle from a spectrally matched
photodiode receiver. During a NO SMOKE condition, only light reflected from
the chamber walls enters the receiver and shows up as a small photocurrent. As
smoke particles enter the sensing chamber and cross the light beam of the LED,
more light reaches the receiver due to scattering. The receiver circuitry converts
this photocurrent 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 (analog to digital) converter. A digital representation of this signal
voltage is then transmitted to the fire alarm panel for further processing.
These devices use a small radiation source, Americium-241, which emits alpha
particles that ionize air molecules between two electrically charged electrodes.
With the application of a DC voltage to these electrodes, a small ionization
current flows within the chamber. As smoke enters the chamber, a decrease in
ionization current results. This current is converted into a signal voltage by a
transimpedance 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 an
8-bit A to D (analog to digital) converter. A digital representation of this signal
voltage is then transmitted to the fire alarm panel for further processing.
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