Smoke Detectors

The working principle of light scattering smoke detectors is as follows. When the sizes of small particles are greater than the wave length of the incident radiation, the light is scattered in different directions. Known as the ‘Tyndall Effect’, named after its discoverer, this is used for smoke in several different arrangements, however, all depend on light being scattered by smoke onto a photoelectric cell that is obscured from the light beam in normal circumstances.

In the arrangement shown above, the smoke detectors are placed in line with the pulsed infrared light source. In the absence of smoke, the masking disk casts a shadow over the circular region of diameter ‘AB’ and prevents light from emitting diode from falling onto the detecting photocell. In the presence of smoke, light is scattered and some of it falls onto the photocell. The evaluation circuit is arranged to trigger an alarm if the threshold value is exceeded for a predetermined number of consecutive pulses.

References

“Fire Safety at Sea” by Dr. James Cowley