Abstract: Burglar alarms utilizing ultrasonic waves in which ultrasonic waves transmitted from a wave transmitting oscillator are reflected by such objects as doors, windows, floors, walls, etc. and such reflected ultrasonic waves will be received by a wave receiving oscillator. Then, when those received ultrasonic waves are disturbed by an illegal intruder, a change in signal will be detected and become an input signal for actuating alarms. Remote units each consisting of an amplifier, wave transmitting oscillator, wave receiving oscillator, etc. will be installed directed at doors, windows, etc., and such remote units are connected to a main unit consisting of a high-frequency voltage generator installed in an watching room or a control room.

Abstract: In burglar alarms in which ultrasonic waves are transmitted into a protected space and reflected to a receiver which detects disturbances in the reflected waves caused by an intruder to actuate the alarm, the alarm might be actuated not only by an illegal intruder but also by a short pulse-type vibration. For such reason, the present invention is so designed not to be actuated by a short vibrations with a timer being inserted so as to be actuated only by an input signal continuing for more than a fixed time (practically 0.7 seconds). Further, when such signal continues for more than a fixed time and contains a sound of a telephone bell containing a high frequency component, the high frequency component controls a switching element, permitting a setting time of the timer to extend over a telephone ringing time, thus arranging the alarms not to operate.