Smoke detectors are classified into two types based on its sensing technology, namely photoelectric/optical and ionization.

Photoelectric smoke detector or optical smoke detector operates using light scattering principle and contains a Light Source (LED) and a light sensor placed inside a chamber. The LED pulses at specified intervals or on appropriate command from the fire alarm panel to check the presence of smoke. In normal conditions the light beam from the light source will not be directed to the light sensor.

When smoke enters the chamber some of the light get scattered and falls on the light sensor (photodiode) . The photodiode converts this light into electrical signals and the intelligent integrated circuit triggers the alarm and alerts the occupants through sounders.