A ceiling-mounted passive infrared detector assembly (30) that has three main components: a junction box (10) which is adapted to be mounted within an opening in a ceiling; a cover (44) adapted to mate with the junction box (10); and an electronic passive infrared detector subassembly (43) including an infrared sensor (42) and processing circuitry (45) for processing electrical signals generated by the infrared sensor (42) and producing output signals. The cover (44) has mounting means such as a flange (48) adapted to mount to the junction box (10) (or directly to a ceiling if a junction box is not used) and an extended portion (50) with a cavity (52) and an aperture (46). The cover (44) also has a lens array (36) located across the aperture (46). The electronic passive infrared detector subassembly (43) is located with respect to the cover (44) such that the infrared sensor (42) is aligned with the lens array (36) when the cover (44) is mounted to the junction box (10) such that the infrared sensor (42) detects infrared energy passed through the lens array (36) from a field of view defined by the juxtaposition of the lens array (36) and the infrared sensor (42).