How to Detect Players and Mobs with Entity Detectors

Place the detector

Mount a Glass Infrared Entity Detector or an Industrial Infrared Entity Detector facing the area you want to watch. For a beam across a corridor, use a Glass Linear Laser Entity Detector or an Industrial Linear Laser Entity Detector and aim it at the opposite wall. The detector emits a redstone signal when an entity crosses its field or beam.

Wire the output

Run redstone dust from the detector's output to an Alarm Lamp, Alarm Siren, Red LED Redstone Indicator, or a Hopper Blocker to stop item flow when someone enters. For a door that opens automatically, wire the detector to a Bistable Glass Switch or Bistable Glass Touch Switch that controls the door.

To distinguish players from other entities, place an Industrial Block Category Detector in the line and configure it to pass only the category you want. To trigger only on a specific floor area, use an Industrial Contact Mat or Industrial Shock Sensitive Contact Mat instead of a beam detector.

For a trap, wire the detector to an Industrial Shock Activated Trapdoor or Rustic Shock Activated Trapdoor so the floor opens when the beam is broken. Test by walking through the beam and confirming the connected block activates.