In one type of
photoelectric device, smoke can block a light beam. In this case, the reduction
in light reaching a photocell sets off the alarm.
In the most common type of photoelectric unit,
however, light is scattered by smoke particles onto a photocell, initiating an
alarm. In this type of detector there is a T-shaped chamber with a
light-emitting diode (LED) that shoots a beam of light across the horizontal
bar of the T. A photocell (light receiving cell), positioned at the bottom of
the vertical base of the T, generates a current when it is exposed to light.
Under smoke-free conditions, the light beam crosses the top of the T in an
uninterrupted straight line, not striking the photocell positioned at a right
angle below the beam. When smoke is present, the light is scattered by smoke
particles, and some of the light is directed down the vertical part of the T to
strike the photocell. When sufficient light hits the cell, the current triggers
the alarm.
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