The PEM7D is a compact entry and approach warning system for use in shops, reception areas, private homes, etc. It is a self contained photoelectric beam and sensor unit, which works by sending out a beam of infrared light, bouncing it of a reflective disc and receiving it on sensor. If the beam is interrupted i.e. when a person walks through it, an alarm is triggered and an audible warning sound can be heard. The time the alarm is on and the volume or both easily adjusted.