The photodetector senses the lack of light and triggers a bell. You can imagine that this same sensor could act as a smoke detector. There would have to be a LOT of smoke before the alarm would go off - the smoke would have to be thick enough to completely block out the light, and that's a lot of smoke. Photoelectric smoke detectors therefore use light in a different way. In the normal case, the light from the light source on the left shoots straight across and misses the sensor.
Photoelectric detectors are better at sensing smoky fires, such as a smoldering mattress.
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