This fire alarm detector is a household photoelecric smoke-sensitive automatic detector. It realizes its function mainly by utilizing the scatter absorption and prevention of the smog particles or mini-particles against the light beam. It s working principle is to detect the smog generated by the fire at the initial stage treatment-identification circuit and special-purpose chips, etc. After the smog enters in the optical dark chamber of the detector, the beam from the ultrared ray source scatters on the surfaces of the smog ions, the light-sensitive diode of the light receptor receives the scattered light and generates a photoelectric signal current which is treated through the photoelectric signal a time retardation to effect the detect-alarming.