Abstract: A dual-frequency keyable control circuit is triggered into providing an electrical control output signal when two passive keying circuits, tuned to the correct radio frequencies are brought close enough to a sensing element of the keyable control circuit to cause mutual influence between the passive keying circuits and the sensing element.When the keyable control circuit detects that modulation is present at both ratio frequencies in sequence, it produces control signals which can unlock a door, actuate or defeat an alarm, or perform other functions for which it is desired to require key combinations to attain access.

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. Electrical interaction between the resonant circuits is accomplished by reactive cancellation. 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.

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 keyable control circuit generates a narrow-band FM signal. A keying circuit, containing two tuned circuits, placed next to the tank coil of the keyable control circuit, absorbs rf energy each time the FM signal sweeps past the resonant frequency of one of the tuned circuits. Detector circuits within the keyable control circuit, upon sensing the amplitude modulation imposed by the cyclic absorption by one of the tuned circuits, cause the rf frequency of the FM signal to jump to a second frequency region. If the second tuned circuit in the keying circuit matches the new frequency, detector circuits again detect the amplitude modulation imposed by the cyclic absorption. After detecting the second signal, the keyable control circuit generates an electrical control signal output for use by external circuits.