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Smoke Detectors
Becoming Aware of Radiation Sources:

Photoelectric Technology

Smoke Detectors

Photoelectric technology smoke alarms use a T-shaped chamber fitted with a light-emitting diode (LED) and a photocell. The LED sends a beam of light across the horizontal bar of the chamber. The photocell sits at the bottom of the vertical portion of the chamber. The photo cell will generate a current, when exposed to light.

The diagram below illustrates how the technology works. Under normal, smoke-free conditions, the LED beam moves in a straight line, through the chamber without striking the photo cell. When smoke enters the chamber, smoke particles deflect some of the light rays, scattering them in all directions. Some of it reaches the photocell. When enough light rays hit the photocell, they activate it. The activated photocell generates a current. The current powers the alarm, and the smoke alarm has done its job.

Photoelectric Smoke Alarm Technology
smoke free chamber
light beam travels straight through;
smoke particles in chamber
deflect some light rays
powers alarm
  no light reaches photoelectric cell deflected light rays
activate photocell


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