The circuit uses a voltage comparator and monostable timer to detect movement and trigger an alarm. It is very sensitive and can detect a moving person from a distance of a few meters in daylight or indoor lighting without precise light beam alignment. When a person crosses the detection area, their shadow causes the light intensity seen by an LDR to change, triggering the comparator. This activates the monostable timer and sounds an alarm. The circuit can be assembled on a PCB and housed in a case with the LDR facing the entrance to monitor an area simply with ambient light detection.