pir detector comparator circuit
Popular one implemented by means of heat detector sensor (I do not know how it is exactly called in english) with mirros reflecting heat to it The mirros define sensor detection area and direction . When moving subject crossses particular mirror area , the heat sensor output value will be changed ,and this will be detected by comparator circuit .
But there are another types , which based on induction parameters change , optical radar based ,
ultrasound radar based (but these are not so popular)...