In a photoelectric spot-type detector, what event triggers the alarm?

• A. A predetermined amount of light collected
• B. A rise in temperature
• C. A detection of gas
• D. A mechanical vibration

Answer: (A)

Photoelectric spot-type detectors work by sending light into a chamber and watching for light that is scattered toward a photodetector. In clean air, little light reaches the sensor. When smoke enters, the particles scatter more light into the detector, and once the collected light reaches a preset threshold, the alarm is triggered. This threshold—having a predetermined amount of light detected—is what sets off the alarm. The other options describe triggers for different types of detectors (temperature rise for heat detectors, gas presence for gas detectors, and vibration for vibration or shock sensors), which aren’t how a photoelectric spot-type unit operates.