CAPACITIVE ALARM SYSTEM

摘要

The present invention relates to a alarm system for surveillance of a container (10) located within an electric or electromagnetic field surrounding said container, said alarm system comprising, a detector device (2) arranged to said container, said detector device comprising, generating means (6) for generating an electric or electromagnetic field, balancing means (8, 10, 12) for building up a balanced electric or electromagnetic field, filtering means (16, 18) to prevent the detector device from being affected by chances in temperature and humidity. The invention is characterized in that the container (10) includes conductive material and connected to said detector device so as to constitute an antenna (4) for sensing small capacitive changes in said electric or electromagnetic field, that the generating means and balancing means respectively generates and builds up an electric or electromagnetic field around said container, said generating means (6) being a square wave generator, the unaffected square wave from said generator means (6) and the square wave capacitively affected by the field surrounding the container (4) being fed to amplifier means (14) to amplify the difference thereof, and defining means for defining at least one level of protection.