How does a photoelectric smoke detector work?
There are two versions of photoelectric detectors, although both work on the same principle. The first type is activated when a light that is shining on a light-sensitive sensor is blocked by incoming smoke. Hence, you cannot see through smoke, and where there is smoke, there is fire. The second type is activated when the light shining past the light-sensitive sensor is reflected onto the sensor by incoming smoke. Hence, you can see smoke and where there is smoke there is fire.