Photoelectric Smoke Detectors Explained
The photoelectric smoke detector was developed by Duane Pearsall and Stanley Peterson in 1965, but did not go into wide use until 1975. In a photoelectric detector, current is generated when the light from a small light source (often an LED) hits a photosensitive unit (photocell) in the detector. When light hits the photocathode, it generates electrons that are, in turn, picked up by an anode. This generates current. (See an electrical diagram of a photoelectric smoke detector here.)
There are two ways a photoelectric detector can be set up: a straight line, in which a decrease in current triggers the alarm, and a T shape, in which the creation of current triggers the alarm. In the first type of photoelectric detector, the alarm sounds when smoke gets between the light and the photocell. In the more common T-shaped detector, the photocell is at the base of the T, and light travels along the top of the T. When smoke enters the detector, the light reflecting off the smoke reaches the photocell, triggering an alarm.
In photo-electric detectors, first current is generated by the interaction of light with a photocell. When the current across any of these circuits changes, it triggers an audible and/or visible alarm.