Abstract: An apparatus is disclosed for the non-contact monitoring of the movement of a person and includes an electrical reference signal generator, an ultrasonic transmitter and an ultrasonic receiver. A preliminary amplifier is coupled to the electronic receiver to receive and to amplify the electrical signal supplied by the ultrasonic receiver and to form an amplified electrical signal of increased amplitude. In order to improve the sensitivity of the apparatus, first and second electrical circuits are provided, with each circuit introducing its own phase shift to the amplified signal from the preliminary amplifier. The total phase shift between the first and second amplified signals is equal to 90°. The apparatus further includes an alarm circuit and a summer circuit or adder which is connected to the electrical circuits and to the alarm circuit. The summer circuit adds the direct current voltages produced by the first and second electrical circuits and amplifies the results.