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.


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.


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.