Smoke Alarms
A smoke detector is a device that senses smoke, typically as an indicator of fire. Commercial security devices issue a signal to a fire alarm control panel as part of a fire alarm system, while residential smoke detectors, also known as smoke alarms, generally issue a local audible or visual alarm from the detector itself.
Smoke can be detected either optically (photoelectric) or by physical process (ionization). Detectors may use either, or both, methods. Three different types of smoke alarms are commercially available: (a) ionization smoke alarms (b) photoelectric smoke alarms and (c) dual-sensor smoke alarms.
- Ionization smoke alarms have a small amount of radioactive material, typically americium-241, between two electrically charged plates, which ionizes the air and causes current to flow between the plates. When smoke enters the chamber, it disrupts the flow of ions, thus reducing the flow of current and activating the alarm.
- Photoelectric smoke alarms aim a light source into a sensing chamber at an angle away from the sensor. Smoke enters the chamber, reflecting light onto the light sensor; triggering the alarm.