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How Do Fire Alarms Work?

Fire alarms are small devices that are triggered off when they sense smoke that enters a special chamber inside them. Read on to know more about this topic.
Debopriya Bose Oct 23, 2018
With home security cameras and burglar alarms we sleep peacefully, with the confidence that we are protected from any unwanted intrusion into our homes. True, these home security systems keep us safe from external intrusion. But what about the threats that lurk inside our houses all the time as we go about doing our daily chores? One such threat is fire.

What is a Fire Alarm?

A fire may break out in one room and then spread stealthily to other rooms without any noise or warning. By the time we realize, the raging flames may have engulfed the whole house.
However, the device that amazingly gives us a warning signal against the break out of a fire is the small alarm that lies inconspicuously attached to the ceiling of a room.
These devices have smoke or heat detectors that sense fire and set off an alarm that lets the residents of the house or building know about such a situation.
The alarm sounds off giving enough time to people to evacuate the building. Hence, since they were invented and popularized, they have been instrumental in saving a large number of lives.


There are mainly two types of smoke alarms (fire alarms) depending upon the type of detector that is used in them. The smoke detectors used are the ionization and the photoelectric detectors. Most fire alarms use either one or both the types.

Ionization Detectors

They have a small quantity of Americanum-241 that serves as the source of ionizing radiation. The alpha particles that are constantly released by Americanum-241 strike with the oxygen and nitrogen atoms in the air. This knocks off electrons from the oxygen and nitrogen atoms resulting in the formation of positively charged ions.
Ionization detectors also have an ionization chamber, which houses two plates about a centimeter apart. The battery that is used to run the alarm applies a low voltage to the plates, due to which one plate gets positively charged and the other gets negatively charged.
The positively charged ions of oxygen and nitrogen gas get attracted to the negative plate, whereas the electrons released from the atoms of these gases move towards the positive plate. This generates a small continuous electric current within the alarm. As long as this current is flowing within the detector, the alarm is not set off.
However, once the circuit is broken, the alarm rings out. In case of a fire, when smoke enters the ionization chamber, the smoke particles get attached to the positively charged ions and neutralize them. This results in a drop in the current that triggers off the alarm.

Photoelectric Detectors

In these devices, the alarm is set off once the straight path of light beam inside the photoelectric chamber gets disrupted by smoke particles. Photoelectric detectors have a T-shaped chamber that contains an incandescent bulb or an infrared LED, which acts as a light source.
There is a lens in these systems that collimates the light being emitted from the light source into a beam. There is a photodiode or some other photoelectric sensor at the base of the vertical arm of the T-shaped photoelectric chamber, which acts as a light sensor.
In the absence of smoke, the beam of light travels in a straight path without coming in contact with the light sensor. However, once the smoke particles enter the photoelectric chamber, the beam of light strikes them and get scattered to hit the sensor. Once the light rays strike the sensor, the alarm is triggered off.

Informative Points

Over 85% of the houses in the United States have fire alarm systems installed in them. Given this fact, the number of fire related casualties should have been way below the recorded statistics. Does this mean that most of the accidents caused due to fire take place in the remaining 15% of the houses that do not have fire alarms?
This interpretation doesn't sound feasible. The fact is that just installing a fire alarm does not guarantee safety from fire breaking out in the house. Once installed, these devices need to be regularly checked if they are in working condition to ensure fire safety.
While some detectors work on batteries, there are others that are powered by the mains electricity of the house. In case of the battery-run fire alarms, one should be aware if the batteries need to be replaced. (They usually last for a year).
In case of systems that run on electricity, it is always advisable to have a battery back up so that the fire alarm can alert the residents of a house in case there is a fire breakout when there is no power supply.
Even though these devices may set off false alarms at times, it is always advisable to check for a fire, in case they are turned on.