Automatic alarm system

Claim 1

An automatic alarm system comprising means for emitting an ultrasonic wave of a given frequency, means for receiving said ultrasonic wave reflected from a moving object to be detected and producing an electrical signal indicative of a shift in the frequency of the received wave from the frequency of the emitted wave, said shift being produced by the vDoppler effect of the moving object upon the emitted wave, clipper means for taking out the upper part of the signal above a given amplitude in the form of a pulse, accumulator means for accumulating said pulse, said accumulator means being operable to produce an output only when at least a preselected number of pulses have been received by said accumulator means within a predetermined time period, an alarm unit responsive to the output of said accumulator means, and a monostable multivibrator connected to receive the pulse from the clipper means, said multivibrator producing an output after a predetermined time delay, a counter connected to receive and count said pulses, said counter producing an output when at least a selected number of pulses have been received, and means responsive only to the concurrent presence of the outputs of both the monostable multivibrator and the counter to operate said alarm unit.