Capacitive detector device and alarm system

An alarm system for surveillance of an object located within an electric or electromagnetic field surrounding said object, said alarm system comprising,

• an antenna (4) for sensing small capacitive changes in said electric or electromagnetic field,
• a detector device (2) connected to said antenna (4) for detecting small capacitive chances in the generated electric or electromagnetic field, said detector device comprising, generating means (6) for generating an electric or electromagnetic field around said antenna (4), balancing means (8, 10, 12) for building up a balanced electric or electromagnetic field around said antenna (4), filtering means (16, 18) to prevent the detector device from being affected bv chances in temperature and humidity, 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 antenna (4) being fed to amplifier means (14) to amplify the difference thereof, and defining means for defining at least one level of protection.