Firewolf FW-2, FW2-H, FW-4, FW4-H advanced photoelectric smoke detectors utilize two bi-colored LEDs for indication of status. In a normal standby condition, the LEDs will flash green every 3 seconds. When the detector senses that its sensitivity has drifted outside the UL Listed sensitivity window, the LEDs will flash red every 3 seconds. When the detector senses smoke and goes into alarm, the status LEDs will latch on red. The units are comprised of an LED light source and silicon photo diode receiving element. In a normal standby condition, the receiving element receives no light from pulsing light source. In the event of fire, smoke enters the detector and light is reflected from the smoke particles to the receiving element. The light received is converted to an electronic signal; these signals are processed in the comparator, and when two consecutive signals exceeding the basic level are received within a specific period of time, the time delay circuit triggers the SCR switch to activate the alarm signal. The status LED lights continuously during the alarm period.