Systems, apparatuses and methods for recognizing or detecting obstacles are provided. Passive infrared (PIR) sensors may be coupled to movable objects, such as unmanned aerial vehicles (UAVs). PIR sensors may detect and recognize obstacles such as humans and determine or calculate a distance to the obstacles. Based on the distance from the movable object to the obstacle, one or more flight response measures such as collision avoidance maneuvers may be effected or implemented.