Smoke Detector

1. What is a photoelectric smoke detector?

Photoelectric Smoke Detector use light to detect fire. Inside the alarm, there’s a light-sensing chamber. In this chamber, an LED light shoots a beam of light in a straight light across the chamber. The alarm detects smoke; when smoke enters the chamber, it deflects the LED light from the straight path into a photosensor in a different compartment in the same chamber. As soon as light beams hit this sensor, the alarm begins to sound.

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Optical smoke detector
1: Optical chamber
2: Cover
3: Case moulding
4: Photodiode (transducer)
5: Infrared LED

2. What is the smoke detector sensing chamber?

The smoke sensing chamber contains an IR led source with a peak spectral emission of 880 nm .This source is placed at an angle of from a spectrally matched photo diode receiver During a no smoke condition,only light reflected from the chamber walls enter the receiver and shows up as a small photo current.

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As the smoke particles enter the sensing chamber  and cross the light beam of LED more light reaches the receiver due to scattering.The receiver circuitry converts this photo current into a signal voltage.In a detector when this voltage reaches a preset level an alarm is produced.In a sensor this signal voltage goes into an 8 bit A to D converter A digital representation of this signal voltage is then transmitted to the fire alarm panel for further  processing.

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