Smoke Detectors Information
Smoke detectors are designed to sense the products of combustion. They are designed for commercial, industrial or residential applications. There are four basic types of smoke detectors:
- Stand-alone devices
- Detection systems or components
- Integrated circuits (ICs)
- Addressable fire protection devices
Stand-alone smoke detectors include a sensor and detector circuitry. They usually contain a built-in alarm, may instead provide electrical contacts for an auxiliary alarm.
Smoke detection systems are networked, programmable products that include a control unit and detector. Individual smoke detection components are also commonly available.
Smoke detectors with an IC form factor are semiconductor devices that are designed for smoke-sensing applications. They do not contain an actual smoke sensor, and may be suitable for AC or DC – but not both.
Addressable smoke detectors are fire detection products that communicate with each other in a networked system.
In terms of technology, smoke detectors provide two choices: ionization chamber and photoelectric.
Ionization detectors use an ionization chamber and a source of ionizing radiation to detect smoke. They are commonly available because they are relatively inexpensive, and are good at detecting the relatively small amounts of smoke produced by flaming fires. Ionizing smoke detectors contain a very small amount of radioactive material, however, and require proper disposal.
Photoelectric smoke detectors contain a light and a sensor that are at 90-degree angles to one another. Normally, the light from a light source on the left moves straight across and misses the sensor. When smoke enters a photoelectric smoke detector, however, smoke particles scatter the light and cause some amount of light to hit the sensor. The sensor then sets off the alarm in the smoke detector. Photoelectric smoke detectors are better at sensing smoky fires, such that of a smoldering mattress.
Smoke detectors have audible alarms, visible alarms such as light-emitting diodes (LEDs), or both. They may also have an alarm output to a control unit. Smoke detectors without a built-in alarm come with alarm contacts for output to an external annunciator in the detector base, or to a control unit. Smoke sensing ICs may have piezoelectric horn drivers or alarm output circuitry. The National Fire Protection Association (NFPA) establishes minimum requirements for both fire alarm systems and initiating devices such as smoke detectors. Smoke detectors and fire alarm systems that support NFPA 72 must produce an audible alarm signal that is different from all other audible signals. NFPA 72 defines performance requirements and basic features for smoke detectors and their proper spacing in commercial, residential, and industrial buildings.