the system includes optical detectors , a control microprocessor , and an alarm . the detectors optically detect the presence of cartons within the shelf exit openings and emit carton - present signals upon such detection the microprocessor is responsive to the signals and activates an alarm when a preselected detector signal pattern is observed . in the preferred embodiment , the alarm patterns include four signals within 15 seconds or one signal having a duration of 6 . 5 seconds .