Patent US 5,377,906

A device for detecting and signalling the presence of an object in a closed container, comprising: PA1 a housing; PA1 one or more sensors each comprising an electronic circuit having a detector and an optical signal trigger, wherein the one or more sensors are coupled to the housing; PA1 a power source electrically connected to the one or more sensors; and PA1 an optical signal electrically connected to the optical signal trigger, and its use in providing an electronic mailbox which allows an observer to detect the presence of mail from a remote location.