How Do Smoke Alarms Work?

When you glance at the house roofs, offices and walls, you must have found some plastic encasing installed on the walls. Well, they are nothing but the smoke alarms. The design of a smoke detector is simple to understand.

Generally, there are two types of smoke alarms - the optical detectors and ionization alarms. Knowing the working of these detectors will give help us understand the working of smoke alarms.

An optical smoke alarm uses the principle of light sensors to identify the smoke and hence, detect fire. It consists of LED lights (light emitting diode), a lens and a photodiode. Let's know how light helps to detect a fire.

A light emitting diode is a semiconductor device that operates on the principle of a diode. When the diode is forward biased, recombination of holes and electrons take place, thereby releasing energy in the form of light.

In normal conditions (when there is no fire or smoke), light from the light emitting diode passes the detector in a straight line and so the alarm is not triggered. A lens is provided in the optical smoke alarm device to direct the light source in the form of a light beam.

In the events of a smoke, the smoke particles fill the smoke alarm and due to the presence of the smoke particles, the light beam gets deflected from the straight line path. The deflected light beam strike the photoelectric sensors (photodiodes) and thus the alarm is triggered.

You must be thinking what is a photodiode? Well, a photodiode is another form of diode that become active when a light beam (photons) falls on it. These alarms are also called photoelectric alarms.

In the ionization smoke alarms two small metal plates are located at a distance of 1 cm. These metal plates are termed as electrodes and they maintain a certain potential difference between them.

Generally, this entire set-up is called an ionization chamber. The ionization chamber consists of a radioactive material Americium 241. This material is a good source of alpha particles.

The alpha particles emitted by the Americium 241 collide with the molecules of air (mostly nitrogen and oxygen) and ionize them, i.e. they make them charged particles from neutral molecules. This creates ions in the chamber.

Now, these ions and electrons are attracted towards the electrode plates and hence, a potential difference is created between them. This leads to a flow of charge in the ionization chamber. Now, when smoke particles enter the enclosure, they obstruct the interaction of these ions and electrons and so there is an appreciable drop in the current.

This drop in the current reading triggers the alarm. Some problems with ionization smoke detector is that they emit a small amount of radioactive radiation. However, it is believed that they are generally harmless and can be used in houses.

Smoke alarms are one of the most popular fire alarm systems. Smoke alarms must be installed in every room of the house and connected by a 3-way wire that interconnects all the smoke alarms. This will set an alarm throughout the house even when the fire occurs in only one room.

When you select a fire alarm make sure that the sound emitted by the alarm is loud enough to warn you about the threat of a fire. Generally, smoke alarms work on electricity and batteries. Battery operated smoke alarms are usually used in homes, that last for over a period of one year.