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.