Abstract

An electronic security system utilizing a transmitter and receiver in combination with a resonant tag circuit which is responsive to at least one frequency of electromagnetic radiation. The transmitter provides electromagnetic radiation in a predetermined area at a frequency close to the resonant frequency of the resonant tag circuit. The transmitter additionally modulates its frequency with a detection modulation frequency Fd. The receiver is responsive to electromagnetic radiation and picks up at least a component of the detection modulation frequency Fd. In a preferred embodiment, this component is an AM signal radiated by the resonant tag circuit when the tag circuit is in turn activated by the frequency modulated transmitter frequency. A detection logic circuit responsive to the presence of AM signal at frequency Fd, signals an appropriate alarm. Additional embodiments utilize varying transmitter frequencies and numbers of resonant frequencies in each tag to permit wide application of the security system not only to prevent theft or surreptitious removal of objects but also to permit rapid effective identification of individuals and to provide access to security areas to properly identify individuals.