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How Do Smoke Detectors Work

Smoke detectors, also called smoke alarms, are small appliances that are installed in commercial and residential buildings to warn inhabitants of fire. As their name suggests they are triggered by smoke. They come in a variety of different types with some being battery powered while others are wired into the buildings electric system. Most commercial and mass residential smoke detectors trigger a fire system which alerts the local fire authorities of a potential fire. Household smoke alarms issue a loud auditory alarm or a visual alarm to warn occupants of the potential danger.

How do smoke alarms work?
Smoke alarms are either photoelectric (optical detection) or work using ionization (a physical process). Some smoke detectors combine both systems to make them more sensitive and are often used in public toilets and schools to deter smoking. Smoke alarms have two parts, a sensor to detect smoke and a loud electronic horn.

Photoelectric smoke detector
In a photoelectric smoke alarm the sensor makes use of light to detect smoke. Inside the smoke detector a light, usually an LED globe, and a sensor are placed at a 90 degree angle to each other. Under normal circumstance the light shines straight across and does not hit the sensor. When the particles of smoke enter the smoke detector they cause the light to refract and scatter which enables the light to hit the sensor plate. This in turn sets off the smoke alarm.

Ionization smoke detector
The detector inside the ionization smoke alarm is made up of two metal plates or electrodes about 1 cm apart with a voltage running across them and a small amount of ionizing radiation (from americium-241). This is called an ionization chamber. The radiation ionizes (To “ionize” means to “knock an electron off of”) the oxygen inside the smoke alarm inside of the ionization chamber. The electric current passes between the two plates and the electronics inside the smoke alarm detect this current. When smoke enters the chamber it causes a disruption in the electric current. This triggers the smoke detector to activate and sets off the alarm.

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