Dual frequency alarm device using Doppler effect for sensing a moving object

摘要

An electrical or supersonic wave having a predetermined frequency is emitted as a monitor wave from an emitting device into a zone to be monitored. When a moving object having an inherent velocity range intrudes into the zone, a reflected wave of the monitor wave from the moving object is received by a receiver and an alarm signal is issued when the frequency deviation of the reflected wave relative to the frequency of the emitted monitor wave is within the frequency range corresponding to the velocity range. The alarm device has a circuit for producing a first alarm responsive to the alarm signal, a circuit for changing the frequency of the monitor wave when the first alarm is issued, and a circuit for producing a second alarm when a further alarm signal is obtained from the monitor wave with changed frequency within a predetermined time after change of the monitor wave frequency. The circuit for changing the frequency of the monitor wave comprises a variable capacitance diode and a voltage regulating circuit which progressively changes the reverse bias voltage for the variable capacitance diode. The circuit for producing the second alarm comprises a signal processing circuit that produces a trigger signal for producing the most important alarm when an alarm signal has been produced for the overall frequency relative to the monitor wave with progressively changing frequency. The alarm is designed for inclusion of different degrees of seriousness of warning by gradual reduction of the monitor zone with changes in the frequency of the monitor wave.