How does a smoke alarm work?

Smoke reflects light that can be measured. Within the smoke alarm is the so-called "smoke chamber". This chamber contains an infrared diode that permanently emits light signals. These signals don't come into contact with the light-sensitive photoelectric cells until smoke particles enter the chamber.

If smoke particles do spread into the sensing chamber, as in the case of fire, they reflect the light from the infrared diode in such a manner that the light impulses appear before a highly sensitive photo element. This effect is interpreted as a fire hazard by the electronic engineering and an alarm is triggered.