A smoke detector, also called a smoke alarm, is a safety device that gives out a warning sound whenever it senses or contacts smoke. The audio signal warns the people in the area where the detector is situated, thereby helping them escape catastrophes, such as fire accidents. These smoke detectors could be affixed to the ceilings or walls of bedrooms, hallways, living rooms, etc. in a house.

A smoke detector affixed to the ceiling. Image credit: Flickr
A smoke detector affixed to the ceiling. Image credit: Flickr

Generally, a smoke alarm is mandatory in any house that has a water heater, furnace, cooktop, grill, or range. Different rooms in a house must have their own detectors, as smoke does not usually spread from one area to another area in a house, particularly if the house has multiple storeys. Therefore, a multi-storey house or building must have at least one smoke detector for every floor. For complete protection, the different smoke detectors must be interconnected and not used as solitary alarms so that multiple alarms could trigger simultaneously when there’s impending trouble.

In a commercial or industrial setup, multiple smoke detectors may be needed for a single room, since the floor area could be much bigger. Quite often, organizations or companies could have these smoke detectors supplementing a much bigger fire alarm system.


A smoke detector is usually placed on or closer to the ceiling; preferably close to a stairway, doorway or window – the routes through which smoke usually tries to escape. These detectors should be a part of any room where people sleep, walk or spend most of their times. When humans sleep, they don’t smell smoke and are invariably woken up by heat from the fire. A smoke alarm helps wake them up much before it’s too late.

The smoke detector shouldn’t be placed in the following conditions/locations:

  • Outdoors or any place where there’s direct sunlight
  • Near a cooking appliance – there must at least be a gap of five feet between the detector and the appliance
  • Close to a fuel-burning source or appliance – for instance, there should be a distance of 20 feet or more from a water heater or furnace
  • High-humidity areas, such as a shower or bath, humidifier, sauna, dishwasher or vaporizer
  • Kitchen, garage, furnace space, or any area that’s extremely dirty, dusty or greasy
  • Areas with extremely cold or hot weather conditions – 40 degrees Fahrenheit to 100 degrees Fahrenheit is the ideal temperature

Hard-wired smoke alarms need professional installation. On the other hand, battery-powered alarms can be easily installed, and work even when there’s no electric power. However, the batteries should be replaced once annually; lithium batteries could last longer. The detector gives a signal whenever the battery is losing efficiency.

Components and Types

Typically, a smoke detector is made of two fundamental components: a sensor (for sensing smoke) and an electronic horn or alarm. These detectors could work on a lithium battery, 9-volt battery, or 120-volt home wiring. They are mass-produced and therefore not expensive, despite the invaluable service offered.

Different smoke detectors may have slight variations, but the majority are either of the two: a photoelectric detector, or an ionization detector.

Photoelectric Detector

Also called an optical detector, the photoelectric detector uses the principle of light scattering. It comprises an infrared LED and photodiode light receptor, which reside within the lightproof chamber cover of the detector. The LED emits a stream of light every 10 seconds into the chamber to detect presence of smoke particles. Usually, during a fire breakout, smoke enters the sensor chamber through the detector’s vents. The vent is usually too small for insects or any other solid materials to enter. Upon entering the chamber, the smoke disrupts the infrared light beam. As a result, the light scatters and contacts the photodiode light receptor. A signal is then received by the integrated circuit, causing the alarm sound.

Ionization Detector

Powered by batteries, an ionization detector comprises an air-filled section where a couple of thin, small electricity-conducting wires (electrodes) are installed. These are positioned a centimeter apart, with one electrode having a positive and another a negative charge. There’s also an ionizing radiation source – an extremely small portion (almost 1/5000th of one gram) of americium-241 (an americium isotope), which is an alpha particles or helium nuclei source.

The americium-released alpha particles constantly disband the electrons of the atoms present in the air, thereby converting the chamber’s nitrogen and oxygen atoms into ions. The positive charge causes the nitrogen and oxygen atoms attach to the negatively charged electrode, and the electrons attract to the positive electrode. This results in a small, but continuous flow of electric current. When there is no smoke, an electric current constantly passes between these electrodes. When smoke gets into the section, the flow of electric current is disrupted, causing the device’s alarm to trigger.

The Better Detector: Photoelectric or Ionization?

Both variants have their positives and negatives and can be used interchangeably. For instance, they are susceptible to high humidity or steam, which could result in condensation on the devices’ sensors and circuit boards, causing the alarms to go off. However, a photoelectric detector is more ideal in areas prone to smoldering fires. Compared to an ionization detector, photoelectric detectors are a bit more expensive.

An ionization detector is less expensive and therefore much more common than photoelectric detectors. Moreover, it’s more efficient at detecting smaller quantities of smoke than a photoelectric detector, thanks to its extra-sensitive detection abilities. However, the excessive sensitivity could often cause false alarms. Most homeowners, therefore, turn the detector off when cooking. On the positive side, ionization detectors come built-in with a battery security feature. In other words, when the detector’s battery starts to die, the alarm goes off prompting a battery replacement. There is no such feature in the photoelectric variant.

If you’d like your sensor to detect both minute smoke particles and major fires, you could try a dual-sensor smoke detector. These smoke alarms combine photoelectric and ionization technology, helping you save the costs and hassle of installing two individual smoke detectors. Alternatively, both photoelectric and ionization detectors can be installed.