How Smoke Alarms Work

You may wonder how your smoke alarm works, especially when it’s beeping really loudly at you and there’s no fire you don’t know how to get it to stop except to pull out the battery. Here’s a little information on what’s going on there.

First off, there are two different types of smoke alarms engineered for home use: ionization detectors and photoelectric detectors.

ionizationgleebionization2The ionization detectors have specific ionization chambers built inside them that have two plates and a source of ionizing radiation. The alarm’s battery sends a voltage to the plates which makes one possess a positive charge and one possesses a negative charge. Ionization smoke detectors also possess a radiation source, which is generally 1/5000th of a gram of an isotope called Americium-241, which decays and emits alpha particles, or subatomic particles made of two protons and two neutrons. The isotope was chosen because it emits these particles at a very reliable and constant rate, plus the isotope doesn’t have sufficient penetrative power to get through the plastic of the detector’s outer casing. Back to the subatomic particles: as they travel through the ionization chamber they ionize, meaning an electron breaks free from the atoms of oxygen and nitrogen that are also in the chamber. These freed electrons have a negative charge and are attracted to the aforementioned positively charged plate, while the atoms that lost the electron are not positively charged and will be attracted to the negatively charged plate. This will create a small but constant current between the two plates, which will only be blocked given that smoke enters the chamber and obstructs the ionization and the consistency of the current. When this current is interrupted, the alarm is triggered.

There are a few setbacks to the technology behind ionization detectors. Their possession of a radioactive isotope makes it more difficult to properly dispose of the detectors once you’re through with them, and their extremely sensitive design makes them prone to false alarms when dust, steam and other vapors enter the casing. That said, they’re built with hot, fast fires that emit little smoke in mind, so they can be relied upon given an emergency.

smoke detector2The other common type of detector is called a photoelectric detector, which contains a light-emitting diode (LED) that sends a beam of light across the top of a T-shaped chamber, at the base of which is a photocell that detects light. A diode is a semiconductor device¬†with two terminals that typically allows current to flow only in one direction. Anyway, when smoke enters the chamber, it scatters the light emitted by the LED so that it’s able to hit the photocell (normally the light would just cross from one end of the cross of the T to the other, never reaching the base). If a certain amount of light hits the photocell because of smoke, it triggers an electric current that will ultimately set off the alarm. These alarms are designed to detect slow and smokey fires and are not as sensitive or prone to false alarms as the ionization design.

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