Smoke Detector

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How Does Optical Smoke Detector Work?

Smoke Detector – An optical smoke alarm (also called photo-electric smoke alarm) works using the light scatter principle,. The alarm contains a pulsed Infra red LED which pulses a beam of light into the sensor chamber every 10 seconds to check for smoke particles.

When a fire breaks out smoke will enter the optical chamber through the opening vents. Smoke alarms from quality manufacturers have the chamber protected with insect screens to stop bugs entering and causing false alarms.

As the smoke enters the optical chamber, its particles cause the Infra red light to be scattered onto the photodiode light receptor.

Once the scattered light hits the photodiode light receptor a signal is sent to the integrated circuit which causes the alarm to sound alerting the occupants to the fire. 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 strikes 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.

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When mounted on a flat ceiling, smoke detection devices have an individual coverage of 7.5m radius. However these radii must overlap to ensure there are no “blind spots”. Therefore individual coverage can be represented by a square measuring 10.6 X 10.6m giving an actual coverage area of 112 m² per device.

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Introduction to Optical Smoke Detectors

An optical smoke detector (also called a photoelectric smoke alarm) is a fire detection device that uses light scatter technology to sense the presence of smoke particles. It is a key component of a fire detection and alarm system, offering early warning to prevent fire hazards.

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Working Principle of an Optical Smoke Detector

The smoke detector contains a pulsed infrared LED that emits a beam of light into the sensor chamber every 10 seconds to check for the presence of smoke particles.

1. Normal Condition:
   • In the absence of smoke, the light beam travels in a straight line without hitting the photodiode light receptor.
   • The smoke sensor remains inactive, and no alarm is triggered.
2. During a Fire Event:
   • When smoke enters the optical chamber through the vents, it causes the infrared light to scatter.
   • The scattered light hits the photodiode receptor, generating a signal.
   • This signal activates the alarm, alerting the occupants of the fire.

Quality manufacturers design smoke detectors with insect screens to prevent false alarms caused by bugs entering the chamber.

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Key Components of an Optical Smoke Detector

• LED (Light Emitting Diode):
  • A semiconductor device that emits light when forward biased.
  • During normal conditions, the light beam passes uninterrupted through the detector.
• Lens:
  • Directs the light beam in a straight path.
  • When smoke particles enter, they scatter the light, diverting it toward the photoelectric sensor.
• Photodiode Sensor:
  • A light-sensitive semiconductor that activates when photons strike it.
  • Upon detecting scattered light, it sends a signal to the fire control panel, triggering the alarm.

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Coverage and Installation Guidelines

When mounted on a flat ceiling, a smoke detector covers a 7.5m radius. To eliminate blind spots, these radii should overlap.

• Coverage area:
  • Each fire detector covers 112 m², represented by a square measuring 10.6m x 10.6m.
  • Proper placement ensures complete coverage and effective fire detection.

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Fire Alarm and Control Panel Integration

In large facilities, optical smoke detectors are connected to a fire alarm system and a fire control panel.

• The control panel receives signals from multiple smoke sensors, helping detect fires early.
• It activates alarm systems, such as sirens, strobe lights, or emergency notifications.

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Optical smoke detectors play a vital role in fire safety by using light scatter technology to detect smoke effectively. They are essential for residential, commercial, and industrial fire alarm systems, ensuring early detection and minimizing fire-related damages.