Passive-type infrared sensor system for detecting human body

A passive-type infrared sensor system for detecting a human body, which comprises:

• first and second sensor units each including a light receiving element for outputting an electric output signal proportional to an amount of change of infrared energies incident upon the light receiving element, and an optical system for collecting and guiding the infrared light from a predetermined watch area onto the light receiving element;
• said first sensor unit being so disposed as to define a first watch area clear of a ground surface and oriented towards an upper half of the human body whereas said second sensor unit is so disposed as to define a second watch area below the first watch area and oriented towards a point on the ground surface spaced a predetermined watch distance away from the site where the second sensor unit is located;
• first and second level detecting circuits each operable to output a detection signal only when the electric output signals generated from the light receiving elements of the respective first and second sensor units exceed a predetermined level;
• and
• a human detecting circuit for outputting a human detection signal only when the detection signals are outputted respectively from the level detecting circuits, wherein each of the first and second level detecting circuits outputs one at a time a detection signal and a non-detection signal which are binary signals, and said human detecting circuit comprises an AND gate circuit operable in response to receipt of the binary signals.