Photoelectric Detectors can also act as a smoke detector. If it ever got smoky enough to block the light beam, the bell would go off. But there are two issues here: It's a pretty big smoke detector, and it's not very sensitive. There would have to be a lot of smoke to trigger the alarm - the smoke would have to be thick enough to completely block out the light.

Photoelectric smoke detectors use a different way of measuring light. Inside the smoke detector there is a light and a sensor, but they are positioned at 90-degree angles to one another, like a 'T'. In the normal case, the light from the light source on the left shoots straight across the top of the 'T' and misses the sensor located in the bottom of the 'T'. When smoke enters the chamber, however, the smoke particles scatter the light and some amount of light hits the sensor: The sensor then sets off the alarm in the smoke detector. Photoelectric detectors are better at sensing smoky, smoldering fires.