The smoke sensor is a T-shaped chamber with an infrared
LED placed in the left opening that emits a beam of light
across the horizontal portion of the chamber. The right
opening is open for smoke particles to enter as base opening
for the phototransistor. The phototransistor generates a
current when exposed to light. If there is no smoke present
inside the chamber, the beam of light is emitted across the top
of the T-shaped chamber. Thus, the phototransistor does not
generate current as no light has hit it. When smoke particle
enter the T-shaped chamber, however the beam of light is
scattered by smoke particles. Subsequently, some of the light
is directed down into the vertical portion of the T-shaped
chamber and strikes the phototransistor.