Patent ID: 6943685

Claim:
An intrusion detecting device which comprises: first and second sensor units having respective non-overlapping distant and near alert areas defined from a site distant from and to another site near to a subject to be monitored for security purpose and operable to detect an intruder entering distant and near alert areas, respectively; a sound generating device capable of generating one at a time, a first warning prerecorded for the distant alert area and a second warning prerecorded for the near alert area, said first and second warnings being different from each other; a first timer circuit counting a first predetermined length of time subsequent to a first detection by the first sensor unit in the distant alert area; and a warning generating unit operable to automatically cause the sound generating device to issue the first warning when the first detection is made by the first sensor unit in the distant alert area, and also to automatically cause the second generating device to issue the second warning when a second detection is made by the second sensor unit in the near alert area during the first predetermined length of time subsequent to the first detection by the first sensor unit.