Smoke detectors detect the presence of smoke particles as an early indication of fire. In general, smoke detectors are based on the principle of detecting smoke particles in the air. Typical smoke detectors function to sense smoke either by sensing atomic particles using an ionization process, or by seeing smoke particles using a photoelectric process. Some smoke detectors work by sensing the presence of particulates in smoke by detecting an interruption in photoelectric energy, some detectors employ the use of an ionization of an air sample to detect smoke. Smoke detectors that detect the products of combustion and sound an alarm when a concentration threshold is exceeded are coming into widespread use. Fire or smoke detecting systems generally include individual detecting units that operate relatively independently of central control. Detection units typically comprise two main components including a smoke chamber and electronics to sample the environment and generate the associated data, and a housing surrounds the chamber/electronics and is typically ornamental in design. The smoke detection units rely on one of a number of different principles to sample the environmental smoke. Typical detectors also require complicated control electronics to detect the light level including analog amplifiers, filters, and comparators. Smoke and fire detectors require electrical energy to operate. Smoke detectors are often battery powered, but may also be supplied from the AC power. Some detectors are solely powered by conventional AC power, some are AC powered but have a back-up battery. A third type of detector is intended to be primarily powered by AC with the back-up battery only providing necessary power to operate the detection and alarm circuitry in the event of loss of AC power.

Various types of smoke detectors are known, including thermal type detectors which detect heat from a fire causing a bimetal to flip over or a pneumatic chamber to expand, photoelectric type detectors which detect light scattered by smoke, ionization type detectors which detect changes in ionization current caused by smoke, beam light obscuration type (separate type) detectors which detect the presence of smoke between a separately arranged light transmitter and receiver, light obscuration type detectors which detect reductions in light permeability due to the presence of smoke, and radiation type detectors like infrared detectors which identify a fire outbreak by detecting heat rays from flames or radiation levels of a flame flicker. Obscuration detectors align the emitter and receiver such that light generated by the emitter shines directly into the receiver. Smoke particles in the test atmosphere interrupt a portion of the beam thereby decreasing the amount of light received by the emitter. Obscuration detectors typically work well for black smoke but are less sensitive to gray smoke. The obscuration sensors in general measure the obscuration or cloudiness of the air. Obscuration may be measured and expressed as the inverse of clarity. Scatter detectors have an emitter and receiver positioned on non-colinear axes such that light from the emitter does not shine directly onto the receiver. Smoke particles in the test atmosphere reflect or scatter light from the emitter into the receiver. Reflected detectors generally work well for gray smoke but have a decreased sensitivity to black smoke. Ionization smoke detectors are prone to nuisance alarms because of their detection of as "smoke", mostly polar molecules including water vapor, moisture, and humidity. Ionization-type detectors respond rapidly to flaming fires. Photoelectric-type detectors respond rapidly to smoldering fires.

While the performance feature offerings among different vendor model smoke detectors may vary, modern smoke detection is usually based on either an ionization or a photoelectric detection technology. Both ionization and photoelectric smoke detectors have proved to be useful in providing warnings of the existence of fire. Both devices respond to the presence of particulate matter or particles of combustion byproducts which, when present in great quantities, are visible as smoke. Both ionization and photoelectric sensors have been located inside housings having complicated vents and baffling designs in order to promote the ingress of smoke. The photoelectric detector responds to changes in light transmission through a medium that contains smoke. The ionization smoke detector responds to changes in the conductivity of an ionized medium due to the presence of combustion products. Ionization smoke detectors are more sensitive than the photoelectric type detectors in detecting smaller particles of combustion, i.e. generally smaller than one micron, which are predominately created by fast flaming fires. Alternatively, photoelectric smoke detectors are more sensitive than ionization detectors in detecting large combustion particulate, i.e. generally larger than one micron, which are created by smoldering fires.

A photoelectric smoke detector measures the ambient smoke conditions of a confined space and activates an alarm in response to the presence of unacceptably high amounts of smoke. Photoelectric detectors include an infrared (IR) light source and an IR photodiode receiver positioned at opposite ends of the detector's chamber. Photoelectric-type smoke detectors quickly respond to relatively large smoke particles generated by smoldering fires. Photoelectric smoke detectors are reasonably effective at detecting grey smoke. Smoke is generally classified as black or grey. Grey smoke particles are generally much easier to detect as they tend to scatter the light from the photoemitting diode very well. The color of the smoke greatly affects the amount of light that is scattered, photoelectric-type smoke detectors respond to black smoke much more slowly than they respond to white smoke. The photoelectric smoke detector detects the generation of smoke by detecting an increase of the amount of the light received by the light receiving element. Photoelectric smoke detection units sample smoke by detecting the changes in the propagation of light through the sample. Scattering-type photoelectric units have a light source and a light sensitive device that are located in a detection chamber. Light emitted by the light source is scattered by smoke particles entering the detection chamber. The incidence of the scattered light on the light detector activates an alarm. In attenuation-type photoelectric smoke detection units, the light emitting diode and light sensitive diode face each other. The level of smoke in the atmosphere is a function of the attenuation of the light reaching the light sensitive diode. A spot photoelectric smoke detector measures smoke conditions at a spot in a spatial region and produces an alarm signal in response to the presence of unacceptably high smoke levels.

An ionization-type smoke detector detects changes in ionization current due to smoke. The ionization type senses changes in the electrical conductivity of the air in a chamber which is subject to radiation. The ionization smoke detector uses a field effect transistor (FET) for detecting a potential change at the joint between the inner and outer ionization chambers and the intermediate electrode is connected to the FET. Response of ionization smoke detectors is based on adsorption of atmospheric molecules on smoke particles. In ionization-type detection units, an isotope radiation source ionizes air samples between two plates, which are held at different potentials. A change in ionic current flow is detected by a detector that activates an alarm indicating the presence of smoke particles. An ionization smoke detector generally comprises a circuit part comprising a printed circuit board on which electronic parts for producing a fire discrimination output when the sensor output reaches a fire discrimination level, a detecting part serving as a sensor for detecting smoke, a fire signal transmitting section for transmitting a fire signal, and a body to which the circuit part and the detecting part are installed.