A smoke detector is a device that detects smoke, typically as an indicator of fire. Commercial, industrial, and mass residential devices issue a signal to a fire alarm system, while household detectors, known as smoke alarms, generally issue a local audible and/or visual alarm from the detector itself.

Smoke detectors are typically housed in a disk-shaped plastic enclosure about 150 millimetres (6 in) in diameter and 25 millimetres (1 in) thick, but the shape can vary by manufacturer or product line. Most smoke detectors work either by optical detection (photoelectric) or by physical process (ionization), while others use both detection methods to increase sensitivity to smoke. Sensitive alarms can be used to detect, and thus deter, smoking in areas where it is banned such as toilets and schools. Smoke detectors in large commercial, industrial, and residential buildings are usually powered by a central fire alarm system, which is powered by the building power with a battery backup. However, in many single family detached and smaller multiple family housings, a smoke alarm is often powered only by a single disposable battery.


An optical detector is a light sensor. When used as a smoke detector, it includes a light source (incandescent bulb or infrared LED), a lens to collimate the light into a beam, and a photodiode  or other photoelectric sensor at an angle to the beam as a light detector. In the absence of smoke, the light passes in front of the detector in a straight line. When smoke enters the optical chamber across the path of the light beam, some light is scattered  by the smoke particles, directing it at the sensor and thus triggering the alarm.

Also seen in large rooms, such as a gymnasium or an auditorium, are devices to detect a projected beam. A unit on the wall sends out a beam, which is either received by a receiver or reflected back via a mirror. When the beam is less visible to the "eye" of the sensor, it sends an alarm signal to the fire alarm control panel.

Optical smoke detectors are quick in detecting particulate (smoke) generated by smoldering (cool, smoky) fires. Many independent tests indicate that optical smoke detectors typically detect particulates (smoke) from hot, flaming fires approximately 30 seconds later than ionization smoke alarms.

They are less sensitive to false alarms from steam or cooking fumes generated in kitchen or steam from the bathroom than are ionization smoke alarms. For the aforementioned reason, they are often referred to as 'toast proof' smoke alarms.


This type of detector is cheaper than the optical detector; however, it is sometimes rejected because it is more prone to false (nuisance) alarms than photoelectric smoke detectors. It can detect particles of smoke that are too small to be visible. It includes about 37 kBq or 1 µCi of radioactive americium-241 (241Am), corresponding to about 0.3 µg of the isotope. The radiation passes through an ionization chamber, an air-filled space between two electrodes, and permits a small, constant current between the electrodes. Any smoke that enters the chamber absorbs the alpha particles, which reduces the ionization and interrupts this current, setting off the alarm.

241Am, an alpha emitter, has a half-life of 432 years. This means that it does not have to be replaced during the useful life of the detector, and also makes it safe for people at home, since it is only slightly radioactive. Alpha radiation, as opposed to beta and gamma, is used for two additional reasons: Alpha particles have high ionization, so sufficient air particles will be ionized for the current to exist, and they have low penetrative power, meaning they will be stopped by the plastic of the smoke detector and/or the air. About one percent of the emitted radioactive energy of 241Am is gamma radiation.


An air-sampling smoke detector is capable of detecting microscopic particles of smoke. Most air-sampling detectors are aspirating smoke detectors, which work by actively drawing air through a network of small-bore pipes laid out above or below a ceiling in parallel runs covering a protected area. Small holes drilled into each pipe form a matrix of holes (sampling points), providing an even distribution across the pipe network. Air samples are drawn past a sensitive optical device, often a solid-state laser, tuned to detect the extremely small particles of combustion. Air-sampling detectors may be used to trigger an automatic fire response, such as a gaseous fire suppression system, in high-value or mission-critical areas, such as archives or computer server rooms.

Most air-sampling smoke detection systems are capable of a higher sensitivity than spot type smoke detectors and provide multiple levels of alarm threshold, such as Alert, Action, Fire 1 and Fire 2. Thresholds may be set at levels across a wide range of smoke levels. This provides earlier notification of a developing fire than spot type smoke detection, allowing manual intervention or activation of automatic suppression systems before a fire has developed beyond the smoldering stage, thereby increasing the time available for evacuation and minimizing fire damage.

Carbon monoxide and carbon dioxide detection

Some smoke alarms use a carbon dioxide sensor or carbon monoxide sensor in order to detect extremely dangerous products of combustion. However, not all smoke detectors that are advertised with such gas sensors are actually able to warn of poisonous levels of those gases in the absence of a fire.

Commercial smoke detectors

An integrated locking mechanism for commercial building doors. Inside an enclosure are a locking device, smoke detector and power supply.

Commercial smoke detectors are either conventional or analog addressable, and are wired up to security monitoring systems or fire alarm control panels (FACP). These are the most common type of detector, and usually cost a lot more than a household smoke alarms. They exist in most commercial and industrial facilities, such as high rises, ships and trains. These detectors don't need to have built in alarms, as alarm systems can be controlled by the connected FACP, which will set off relevant alarms, and can also implement complex functions such as a staged evacuation.


The word Conventional is slang used in to distinguish the method used to communicate with the control unit from that used by addressable detectors whose methods were unconventional at the time of their introduction. So called “Conventional Detectors” cannot be individually identified by the control unit and resemble an electrical switch in their information capacity. These detectors are connected in parallel to the signaling path or (initiating device circuit) so that the current flow is monitored to indicate a closure of the circuit path by any connected detector when smoke or other similar environmental stimulus sufficiently influences any detector. The resulting increase in current flow is interpreted and processed by the control unit as a confirmation of the presence of smoke and a fire alarm signal is generated.


This type of installation gives each detector on a system an individual number, or address. Thus, addressable detectors allow an FACP, and therefore fire fighters, to know the exact location of an alarm where the address is indicated on a diagram.

Analog addressable detectors provide information about the amount of smoke in their detection area, so that the FACP can decide itself, if there is an alarm condition in that area (possibly considering day/night time and the readings of surrounding areas). These are usually more expensive than autonomous deciding detectors.

Tandalone smoke alarms

The main function of a standalone smoke alarm is to alert persons at risk. Several methods are used and documented in industry specifications published by Underwriters Laboratories[14] Alerting methods include:

* Audible tones
o usually around 3200 Hz due to component constraints (Audio advancements for persons with hearing impairments have been made; see External links)
o 85 dBA at 10 feet
* Spoken voice alert
* Visual strobe lights
o 110 candela output
* Tactile stimulation, e.g., bed or pillow shaker (No standards exist as of 2008 for tactile stimulation alarm devices.)

Some models have a hush or temporary silence feature that allows silencing without removing the battery. This is especially useful in locations where false alarms can be relatively common (i.e. due to "toast burning") or users could remove the battery permanently to avoid the annoyance of false alarms, but removing the battery permanently is strongly discouraged.

While current technology is very effective at detecting smoke and fire conditions, the deaf and hard of hearing community has raised concerns about the effectiveness of the alerting function in awakening sleeping individuals in certain high risk groups such as the elderly, those with hearing loss and those who are intoxicated.[15] Between 2005 and 2007, research sponsored by the United States' National Fire Protection Association (NFPA) has focused on understanding the cause of a higher number of deaths seen in such high risk groups. Initial research into the effectiveness of the various alerting methods is sparse. Research findings suggest that a low frequency (520 Hz) square wave output is significantly more effective at awakening high risk individuals. Wireless Wi-Safe smoke and carbon monoxide detectors linked to alert mechanisms such as vibrating pillow pads, strobes and remote warning handsets have been found to support the groups above.

Gas Suppression

Foam Systems