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 household smoke detectors, also known as smoke alarms, generally issue a local audible or visual alarm from the detector itself.
Smoke detectors are housed in plastic enclosures, typically shaped like a disk about 150 millimetres (6 in) in diameter and 25 millimetres (1 in) thick, but shape and size vary. Smoke can be detected either optically (photoelectric) or by physical process (ionization), detectors may use either, or both, methods. Sensitive alarms can be used to detect, and thus deter, smoking in areas where it is banned. 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. Domestic smoke detectors range from individual battery-powered units, to several interlinked mains-powered units with battery backup; if any unit detects smoke, all trigger even in the absence of electricity.
- 1 History
- 2 Design
- 3 Interconnected systems
- 4 Domestic
- 5 Standards
- 6 Legislation
- 7 References
- 8 External links
The first automatic electric fire alarm was patented in 1890 by Francis Robbins Upton, an associate of Thomas Edison. George Andrew Darby patented the first European electrical heat detector in 1902 in Birmingham, England. In the late 1930s Swiss physicist Walter Jaeger tried to invent a sensor for poison gas. He expected that gas entering the sensor would bind to ionized air molecules and thereby alter an electric current in a circuit in the instrument. His device did not meet its purpose: small concentrations of gas had no effect on the sensor's conductivity. Frustrated, Jaeger lit a cigarette and was soon surprised to notice that a meter on the instrument had registered a drop in current. Smoke particles from his cigarette had apparently done what poison gas could not. Jaeger's experiment was one of the advances that paved the way for the modern smoke detector. In 1939 Swiss physicist Ernst Meili devised an ionisation chamber device capable of detecting combustible gases in mines. He also invented a cold cathode tube that could amplify the small signal generated by the detection mechanism to a strength sufficient to activate an alarm.
Ionisation smoke detectors were first sold in the United States in 1951; they were used only in major commercial and industrial facilities in the next several years due to their large size and cost. In 1955 simple home "fire detectors" for homes were developed, detecting high temperatures. The United States Atomic Energy Commission (USAEC) granted the first license to distribute smoke detectors using radioactive material in 1963. The first low-cost smoke detector for domestic use was developed by Duane D. Pearsall in 1965, an individual replaceable battery-powered unit that could be easily installed. The "SmokeGard 700" was a beehive-shaped, strong fire-resistant steel unit. The company began mass-producing these units in 1975. Studies in the 1960s determined that smoke detectors respond to fires much faster than heat detectors.
The first single-station smoke detector was invented in 1970 and made public the next year. It was an ionisation detector powered by a single 9-volt battery. They cost about US$125 and sold at a rate of a few hundred thousand per year. Several technological developments occurred between 1971 and 1976, including the replacement of cold-cathode tubes with solid-state electronics, which greatly reduced the detectors' sizes and made it possible to monitor battery life. The previous alarm horns, which required specialty batteries, were replaced with horns that were more energy-efficient, enabling the use of commonly available sizes of batteries. These detectors could also function with smaller amounts of radioactive source material, and the sensing chamber and smoke detector enclosure were redesigned for more effective operation. The rechargeable batteries were often replaced by a pair of AA batteries along with a plastic shell encasing the detector. The 10-year-lithium-battery-powered smoke alarm was introduced in 1995.
The optical smoke detector was invented Donald Steele and Robert Emmark of Electro Signal and patented in 1972.
An ionization smoke detector uses a radioisotope, typically americium-241, to ionize air; a difference due to smoke is detected and an alarm is generated. Ionization detectors are more sensitive to the flaming stage of fires than optical detectors, while optical detectors are more sensitive to fires in the early smouldering stage.
The smoke detector has two ionization chambers, one open to the air, and a reference chamber which does not allow the entry of particles. The radioactive source emits alpha particles into both chambers, which ionizes some air molecules. There is a potential difference (voltage) between pairs of electrodes in the chambers; the electrical charge on the ions allows an electric current to flow. The currents in both chambers should be the same as they are equally affected by air pressure, temperature, and the ageing of the source. If any smoke particles enter the open chamber, some of the ions will attach to the particles and not be available to carry the current in that chamber. An electronic circuit detects that a current difference has developed between the open and sealed chambers, and sounds the alarm. The circuitry also monitors the battery used to supply or back up power, and sounds an intermittent warning when it nears exhaustion. A user-operated test button simulates an imbalance between the ionization chambers, and sounds the alarm if and only if power supply, electronics, and alarm device are functional. The current draw of an ionization smoke detector is low enough for a small battery used as sole or backup power supply to be able to provide power for months or years without the need for external wiring.
Ionization smoke detectors are usually cheaper to manufacture than optical detectors. They may be more prone to false alarms triggered by non-hazardous events than photoelectric detectors, and have been found to be much slower to respond to typical house fires.
Americium-241 is an alpha emitter with a half-life of 432.6 years. Alpha particle radiation, as opposed to beta (electron) and gamma (electromagnetic) radiation, 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, safely, by the plastic of the smoke detector or the air. About one percent of the emitted radioactive energy of 241Am is gamma radiation. The amount of elemental americium-241 is small enough to be exempt from the regulations applied to larger sources. It includes about 37 kBq or 1 µCi of radioactive element americium-241 (241Am), corresponding to about 0.3 µg of the isotope. This provides sufficient ion current to detect smoke, while producing a very low level of radiation outside the device.
The americium-241 in ionizing smoke detectors poses a potential environmental hazard, albeit a very small one. Disposal regulations and recommendations for smoke detectors vary from region to region. The amount of radioactive material contained in ionizing smoke detectors is very small and thus doesn't represent a significant radiological hazard. If the americium is left in the ionization chamber of the alarm the radiological risk is insignificant because the chamber acts as a shield to the alpha radiation. A person would have to open the sealed chamber and ingest or inhale the americium for the risk to be significant. The radiation risk of exposure to an ionic smoke detector is much smaller than natural background radiation under normal operation.
Some European countries, like France, and some US states and municipalities  have banned the use of domestic ionic smoke alarms because of concerns that they are not reliable enough as compared other technologies.Where an ionizing smoke detector has been the only detector, fires in the early stages have not always been effectively detected.
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A photoelectric, or optical smoke detector contains a source of infrared, visible, or ultraviolet light (typically an incandescent light bulb or light-emitting diode), a lens, and a photoelectric receiver (typically a photodiode). In spot-type detectors all of these components are arranged inside a chamber where air, which may contain smoke from a nearby fire, flows. In large open areas such as atria and auditoriums, optical beam or projected-beam smoke detectors are used instead of a chamber within the unit: a wall-mounted unit emits a beam of infrared or ultraviolet light which is either received and processed by a separate device, or reflected back to the receiver by a reflector. In some types, particularly optical beam types, the light emitted by the light source passes through the air being tested and reaches the photosensor. The received light intensity will be reduced by absorption due to smoke, air-borne dust, or other substances; the circuitry detects the light intensity and generates the alarm if it is below a specified threshold, potentially due to smoke. In other types, typically chamber types, the light is not directed at the sensor, which is not illuminated in the absence of particles. If the air in the chamber contains particles (smoke or dust), the light is scattered and some of it reaches the sensor, triggering the alarm.
According to the National Fire Protection Association (NFPA), "photoelectric smoke detection is generally more responsive to fires that begin with a long period of smoldering". Studies by Texas A&M and the NFPA cited by the City of Palo Alto California state, "Photoelectric alarms react slower to rapidly growing fires than ionization alarms, but laboratory and field tests have shown that photoelectric smoke alarms provide adequate warning for all types of fires and have been shown to be far less likely to be deactivated by occupants."
Although photoelectric alarms are highly effective at detecting smoldering fires and do provide adequate protection from flaming fires, fire safety experts and the National Fire Protection Agency recommend installing what are called combination alarms, which are alarms that either detect both heat and smoke, or use both the ionization and photoelectric processes. Some combination alarms may include a carbon monoxide detection capability.
The type and sensitivity of light source and photoelectric sensor, and type of smoke chamber differ between manufacturers.
Carbon monoxide and carbon dioxide detection
Carbon monoxide sensors detect potentially fatal concentrations of carbon monoxide gas, which may build up due to faulty ventilation where there are combustion appliances such as heaters and cookers, although there is no uncontrolled fire outside the appliance.
High levels of carbon dioxide (CO2) may indicate a fire, and can be detected by a carbon dioxide sensor. Such sensors are often used to measure levels of CO2 which may be undesirable but not indicative of a fire; this type of sensor can also be used to detect and warn of the much higher levels generated by a fire. One manufacturer says that detectors based on CO2 levels are the fastest fire indicators, and also, unlike ionization and optical detectors, detect fires that do not generate smoke, such as those fuelled by alcohol or gasoline. CO2 fire detectors are not susceptible to false alarms due to particles, making them particularly suitable for use in dusty and dirty environments.
A presentation by Siemens and the Canadian Fire Alarm Association reports the ionization detector as best at detecting incipient-stage fires with invisibly small particles, fast-flaming fires with smaller 0.01-0.4 micron particles, and dark or black smoke, while more modern photoelectric detectors are best at detecting slow-smouldering fires with larger 0.4–10.0 micron particles, and light-coloured white/grey smoke.
Photoelectric smoke detectors respond faster to fire in its early, smouldering stage (before it breaks into flame). The smoke from the smouldering stage of a fire is typically made up of large combustion particles—between 0.3 and 10.0 µm. Ionization smoke detectors respond faster (typically 30–60 seconds) in the flaming stage of a fire. The smoke from the flaming stage of a fire is typically made up of microscopic combustion particles—between 0.01 and 0.3 µm. Also, ionization detectors are weaker in high air-flow environments, and because of this, the photoelectric smoke detector is more reliable for detecting smoke in both the smoldering and flaming stages of a fire.
In June 2006, the Australasian Fire & Emergency Service Authorities Council, the peak representative body for all Australian and New Zealand fire departments, published an official report, 'Position on Smoke Alarms in Residential Accommodation'. Clause 3.0 states, "Ionization smoke alarms may not operate in time to alert occupants early enough to escape from smouldering fires."
In August 2008, the International Association of Fire Fighters (IAFF, with over 300,000 members in North America) passed a resolution recommending the use of photoelectric smoke alarms, saying that changing to photoelectric alarms, "Will drastically reduce the loss of life among citizens and fire fighters."
In May 2011, the Fire Protection Association of Australia's (FPAA) official position on smoke alarms stated, "Fire Prevention Association Australia considers that all residential buildings should be fitted with photoelectric smoke alarms..."
In December 2011, the Volunteer Fire Fighter's Association of Australia published a World Fire Safety Foundation report, 'Ionization Smoke Alarms are DEADLY', citing research outlining substantial performance differences between ionization and photoelectric technology.
In November 2013, the Ohio Fire Chiefs' Association (OFCA) published an official position paper supporting the use of photoelectric technology in Ohioan residences. The OFCA's position states, "In the interest of public safety and to protect the public from the deadly effects of smoke and fire, the Ohio Fire Chiefs' Association endorses the use of Photoelectric Smoke Alarms … In both new construction and when replacing old smoke alarms or purchasing new alarms, we recommend Photoelectric Smoke Alarms."
In June 2014, tests by the North Eastern Ohio Fire Prevention Association (NEOFPA) on residential smoke alarms were broadcast on the ABC's 'Good Morning America' program. The NEOFPA tests showed ionization smoke alarms failing to activate in the early, smoldering stage of fire. The combination ionization-photoelectric alarms failed to activate until an average of over 20 minutes after the stand-alone photoelectric smoke alarms. This vindicated the June 2006 official position of the Australasian Fire & Emergency Service Authorities Council (AFAC) and the October 2008, official position of the International Association of Fire Fighters (IAFF). Both AFAC and the IAFF recommend photoelectric smoke alarms, but not combination ionization/photoelectric smoke alarms.
Due to the varying levels of detection capabilities between detector types, manufacturers have designed multi-criteria devices which cross-reference the separate signals to both rule out false alarms and improve response times to real fires.
Obscuration is a unit of measurement that has become the standard way of specifying smoke detector sensitivity. Obscuration is the effect that smoke has on reducing sensor visibility, expressed in percent obscuration per unit length; higher concentrations of smoke result in higher obscuration levels.
|Ionization||2.6–5.0% obs/m (0.8–1.5% obs/ft)|
|Photoelectric||0.70–13.0% obs/m (0.2–4.0% obs/ft)|
|Aspirating||0.005–20.5% obs/m (0.0015–6.25% obs/ft)|
|Laser||0.06–6.41% obs/m (0.02–2.0% obs/ft)|
Smoke detectors may be interconnected, either as part of an alarm system, or simply as grouped alarms in a domestic situation.
Interconnected smoke detectors are either conventional or analog addressable, and are connected to security alarm or fire alarm systems controlled by fire alarm control panels (FACP). These are the most common type of detector, and may be significantly more expensive than single-station battery-operated smoke alarms. They are used in most commercial and industrial facilities and other places such as ships and trains, but are also part of some security alarm systems in homes. 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 to distinguish the method used to communicate with the control unit in newer addressable systems. So called "conventional detectors" are smoke detectors used in older interconnected systems and resemble electrical switches in their information capacity. These detectors are connected in parallel to the signaling path 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. In a conventional system, 32 smoke detectors are typically wired together in each zone and a single fire alarm control panel usually monitors a number of zones which can be arranged to correspond to different areas of a building. In the event of a fire, the control panel is able to identify which zone or zones contain the detector or detectors in alarm, but can not identify which individual detector or detectors are in a state of alarm.
An analog addressable system gives each detector an individual number, or address. Addressable systems allow the exact location of an alarm to be plotted on the FACP. In certain systems, a graphical representation of the building is provided on the screen of the FACP which shows the locations of all of the detectors in the building, while in others the address and location of the detector or detectors in alarm are simply indicated.
Analog addressable systems are usually more expensive than conventional non-addressable systems, and offer extra options, including a custom level of sensitivity (sometimes called Day/Night mode) which can determine the amount of smoke in a given area and contamination detection from the FACP that allows determination of a wide range of faults in detection capabilities of smoke detectors. Detectors become contaminated usually as a result of the build up of atmospheric particulates in the detectors being circulated by the heating and air-conditioning systems in buildings. Other causes include carpentry, sanding, painting, and smoke in the event of a fire. Panels can also be interconnected to control a very large number of detectors in multiple buildings. This is most commonly used in hospitals, universities, resorts and other large centres or institutions.
Smaller, less expensive, smoke alarm systems, typically used in a domestic environment, may be individual standalone units, or interconnected. They typically generate a loud acoustic warning signal as their only action. Several detectors (whether standalone or interconnected) are normally used in the rooms of a dwelling. There are inexpensive smoke alarms that may be interconnected so that any detector that triggers sounds all alarms. They are powered by mains electricity, with disposable or rechargeable battery backup. They may be interconnected by wires, or wirelessly.They are required in new installations in some jurisdictions.
- Audible tones
- Spoken voice alert
- Visual strobe lights
- 177 candela output
- Tactile stimulation (e.g. bed or pillow shaker), although no standards existed as of 2008 for tactile stimulation alarm devices.
Some models have a hush or temporary silence feature that allows silencing, typically by pressing a button on the housing, without removing the battery. This is especially useful in locations where false alarms can be relatively common (e.g. near a kitchen), or users might remove the battery permanently to avoid the annoyance of false alarms, losing the advantages of the alarm.
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. Between 2005 and 2007 research sponsored by the United States' National Fire Protection Association (NFPA) focused on understanding the cause of the higher number of deaths 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 smoke and carbon monoxide detectors linked to alert mechanisms such as vibrating pillow pads for the hearing impaired, strobes, and remote warning handsets are more effective at waking people with serious hearing loss than other alarms.
Batteries are used either as sole or as backup power for residential smoke detectors. Mains-operated detectors have disposable or rechargeable batteries; others run only on 9-volt disposable batteries. When the battery is exhausted a battery-only smoke detector becomes inactive; most smoke detectors chirp repeatedly if the battery is low. It has been found that battery smoke detectors in many houses have dead batteries. It has been estimated[when?] that in the UK over 30% of smoke alarms may have dead or removed batteries. In response public information campaigns have been created to remind people to change smoke detector batteries regularly. In Australia, for example, a public information campaign suggests that smoke alarm batteries should be replaced on April Fools' Day every year. In regions using daylight saving time, campaigns may suggest that people change their batteries when they change their clocks or on a birthday.
Some mains-powered detectors are fitted with a non-rechargeable lithium battery for backup with a life of typically ten years, after which it is recommended that the detector be replaced. User-replaceable disposable 9-volt lithium batteries, which last at least twice as long as alkaline batteries, are available.
The US National Fire Protection Association recommends that home-owners replace smoke detector batteries with a new battery at least once per year, when it starts chirping (a signal that its charge is low), or when it fails a test, which the NFPA recommends to be carried out at least once per month by pressing the "test" button on the alarm.
A 2004 NIST report concluded that "Smoke alarms of either the ionization type or the photoelectric type consistently provided time for occupants to escape from most residential fires," and, "Consistent with prior findings, ionization type alarms provided somewhat better response to flaming fires than photoelectric alarms (57 to 62 seconds faster response), and photoelectric alarms provided (often) considerably faster response to smoldering fires than ionization type alarms (47 to 53 minutes faster response)."
Regular cleaning can prevent false alarms caused by the build-up of dust and insects, particularly on optical type alarms as they are more susceptible to these factors. A vacuum cleaner can be used to clean domestic smoke detectors to remove detrimental dust. Optical detectors are less susceptible to false alarms in locations such as near a kitchen producing cooking fumes.
On the night of May 31, 2001, Bill Hackert and his daughter Christine of Rotterdam, New York died when their house caught fire and a First Alert ionisation smoke detector failed to sound. The cause of the fire was a frayed electrical cord behind a couch that smoldered for hours before engulfing the house with flames and smoke. The ionisation smoke detector was found to be defectively designed, and in 2006 a jury in the United States District Court for the Northern District of New York decided that First Alert and its parent company, BRK Brands, was liable for millions of dollars in damages.
Installation and placement
In the United States most state and local laws regarding the required number and placement of smoke detectors are based upon standards established in NFPA 72, National Fire Alarm and Signaling Code. Laws governing the installation of smoke detectors vary depending on the locality. However, some rules and guidelines for existing homes are relatively consistent throughout the developed world. For example, Canada and Australia require a building to have a working smoke detector on every level. The United States NFPA code cited in the previous paragraph requires smoke detectors on every habitable level and within the vicinity of all bedrooms. Habitable levels include attics that are tall enough to allow access. Many other countries have comparable requirements.
In new construction, minimum requirements are typically more stringent. All smoke detectors must be hooked directly to the electrical wiring, be interconnected and have a battery backup. In addition, smoke detectors are required either inside or outside every bedroom, depending on local codes. Smoke detectors on the outside will detect fires more quickly, assuming the fire does not begin in the bedroom, but the sound of the alarm will be reduced and may not wake some people. Some areas also require smoke detectors in stairways, main hallways and garages.
A dozen or more detectors may be connected via wiring or wirelessly such that if one detects smoke, the alarms will sound on all the detectors in the network, improving the likelihood that occupants will be alerted even if smoke is detected far from their location. Wired interconnection are more practical in new construction than for existing buildings.
In the UK the installation of smoke alarms in new builds must comply with British Standard BS5839 pt6. BS 5839: Pt.6: 2004 recommends that a new-build property consisting of no more than 3 floors (less than 200 square metres per floor) should be fitted with a Grade D, LD2 system. Building Regulations in England, Wales and Scotland recommend that BS 5839: Pt.6 should be followed, but as a minimum a Grade D, LD3 system should be installed. Building Regulations in Northern Ireland require a Grade D, LD2 system to be installed, with smoke alarms fitted in the escape routes and the main living room and a heat alarm in the kitchen; this standard also requires all detectors to have a mains supply and a battery back up.
EN54 European standards
Fire detection products have the European Standard EN 54 Fire Detection and Fire Alarm Systems that is a mandatory standard for every product that is going to be delivered and installed in any country in the European Union (EU). EN 54 part 7 is the standard for smoke detectors. European standard are developed to allow free movement of goods in the European Union countries. EN 54 is widely recognized around the world. The EN 54 certification of each device must be issued annually. 
Coverage of smoke and temperature detectors with European standard EN54
|Surface area (square metres)||Type of detector||Height (m)||Ceiling slope ≤20°||Ceiling slope >20°|
|Smax(square metres)||Rmax (m)||Smax (square metres)||Rmax(m)|
|6< h ≤ 12||80||6,6||110||9,6|
|SA ≤30||EN54-5 Clase A1||≤7,5||30||4,4||30||5,7|
|EN54-5 Clase A2,B,C,D,F,G||≤ 6||30||4,4||30||5,7|
|SA >30||EN54-5 Clase A1||≤7,5||20||3,5||40||6,5|
|EN54-5 Clase A2,B,C,D,E,F,G||≤6||20||3,5||40||6,5|
- EN54-7: Smoke detector
- EN54-5: Temperature detector
- SA: Surface area
- Smax (square metres): Maximum surface coverage
- Rmax (m): Maximum radio
Information in "bold" is the standard coverage of the detector. Smoke detector coverage is 60 square metres and temperature smoke detector coverage is 20 square metres. Height from ground is an important issue for a correct protection.
Australia and United States
In the United States, the first standard for home smoke alarms, NFPA 74, was established in 1967. In 1969, the AEC allowed homeowners to use smoke detectors without a license. The Life Safety Code (NFPA 101), passed by the National Fire Protection Association in 1976, first required smoke alarms in homes. Smoke alarm sensitivity requirements in UL 217 were modified in 1985 to reduce susceptibility to nuisance alarms. In 1988 BOCA, ICBO, and SBCCI model building codes begin requiring smoke alarms to be interconnected and located in all sleeping rooms. In 1989 NFPA 74 first required smoke alarms to be interconnected in every new home construction, and in 1993 NFPA 72 first required that smoke alarms be placed in all bedrooms. NFPA began requiring the replacement of smoke detectors after ten years in 1999.  In 1999 Underwriters Laboratory changed smoke alarm labeling requirements so that all smoke alarms must have a manufactured date written in Plain English.
In June 2013 a World Fire Safety Foundation report titled, 'Can Australian and U.S. Smoke Alarm Standards be Trusted?' was published in the official magazine of the Australian Volunteer Fire Fighter's Association. The report brings into question the validity of testing criteria used by American and Australian government agencies when undergoing scientific testing of ionization smoke alarms in smoldering fires.
In June 2010 the City of Albany, California enacted photoelectric-only legislation after a unanimous decision by the Albany City Council; several other Californian and Ohioan cities enacted similar legislation shortly afterwards.
In November 2011 the Northern Territory enacted Australia's first residential photoelectric legislation mandating the use of photoelectric smoke alarms in all new Northern Territory homes.
In June 2013, in an Australian Parliamentary speech, the question was asked, "Are ionization smoke alarms defective?" This was further to the Australian Government's scientific testing agency (the Commonwealth Scientific and Industrial Research Organisation - CSIRO) data revealing serious performance problems with ionization technology in the early, smoldering stage of fire, a rise in litigation involving ionization smoke alarms, and increasing legislation mandating the installation of photoelectric smoke alarms. The speech cited a May 2013, World Fire Safety Foundation report published in the Australian Volunteer Fire Fighter Association's magazine titled, 'Can Australian and U.S. Smoke Alarm Standards be Trusted?' The speech concluded with a request for one of the world's largest ionization smoke alarm manufacturers and the CSIRO to disclose the level of visible smoke required to trigger the manufacturers' ionization smoke alarms under CSIRO scientific testing.
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|Wikimedia Commons has media related to Smoke detector.|
- Smoke alarm research from the national institute of Standards and Technology
- Report from the UL smoke alarm STP research group
- "Teardown of a Photoelectric Smoke Detector". BigClivedotcom (YouTube). 2015-09-25. - Examination of a Generic Brand photoelectric Smoke Detector.