An optical air quality sensor is designed to sense dust particles. An infrared emitting diode and a phototransistor are diagonally arranged into this device, to allow it to detect the reflected light of dust in air. It is especially effective in detecting very fine particles like cigarette smoke, and is commonly used in air purifier systems.
Sensing dust particles can be quite important actually. Being able to detect those very fine particles such as cigarette smoke can be helpful. Using an infrared emitting diode and a phototransistor embedded into it, detection of particles becomes ready. It could be used in air purifying systems.