Smoke detectors react to the visible or invisible particles created by a fire; however, they do not have an audible signal. For an alarm, they need to be connected to a fire alarm system. The two most common types of smoke detectors are photoelectric and ionization. Each of these types reacts to a specific aspect of smoke. Photoelectric detectors use a light source that shines into the detection chamber. A light sensitive receiver is placed at an angle to the light source so only a small amount of light is normally received.