4098-9714 Smoke Detector Head Fire Alarm. TrueAlarm photoelectric sensors use a stable, pulsed LED light source and a silicon photodiode receiver to deliver consistent and accurate low power smoke sensing. Three user selectable sensitivities for special applications are available for each individual sensor, 0.2%, 0.5%, and 1% per foot. Standard sensitivity is 1.25% to 3.1% per foot. The fire alarm control panel runs an algorithm that can vary the sensitivity for normal applications between 1.25% and 3.1% per foot.