Abstract: A dead-oscillator detector in a wide band frequency modulated keyable control circuit averts attempted actuation of the unlocking function by the coupling of untuned energy-absorbing material, such as iron, to a sensing coil. The energy absorbing material, being unresponsive to frequency, reduces the rf energy in the sensing coil approximately uniformly at all swept frequencies. The dead oscillator detector, lacking an ac component in the rf envelope over the entire frequency band, generates an inhibit signal which prevents the unlocking function. When a tuned circuit is properly coupled to the sensing coil, the resulting ac component in the rf envelope provides one required enable signal to unlock circuits.

Abstract: A keyable control circuit sequentially connects a widely swept fm rf signal to sensing coils at a plurality of detection locations. While rf energy is fed to one of the sensing coils, all other sensing coils are gated off. A resonant detector generates a first signal when an external keying circuit containing a tuned circuit, resonant at a correct first frequency, is inductively coupled to one of the sensing coils. A time-gated envelope detector generates a second signal when the external keying circuit contains a second tuned circuit resonant at a correct second frequency. When the first and second signals are simultaneously present, gating circuits generate a control output signal which may be used to unlock a door or perform other functions access to which is restricted to those possessing keys.

Abstract: A keyable control circuit, has sensing coils located in the vicinity of a plurality of locations where lock control is desired. At least one swept high-frequency oscillator, which is connected to each sensing coil, generates an rf signal, rapidly swept over a wide frequency band. When an external keying circuit, containing more than one resonant circuit, each correctly tuned to a predetermined keying frequency, is inductively coupled to a sensing coil, each resonant circuit absorbs rf energy as the oscillator frequency is swept past its resonant frequency. Energy absorption in the external keying circuit induces corresponding reductions in rf energy in the sensing coil as the oscillator frequency is swept past the keying frequencies. Tuned detectors within the keyable control circuit produce a control signal when energy reduction is sensed at each of the predetermined keying frequencies. If correct absorption fails to occur at any one or more of the predetermined frequencies, the control signal is withheld.

Abstract: A coil is encapsulated in a symbol formed of cast epoxy for display on the exterior of an automobile. The coil is connected to and forms part of a security system which is actuated by the inductive coupling of a keying circuit to the encapsulated coil. Two threaded metal rods are partially embedded in the epoxy symbol to enable mounting on the exterior of an automobile.