The signals are coming from at least one adaptor of the movement detector, and the adaptor has pref. two ultrasonic transducers, each of which is a sender and a receiver and is excited by an oscillator. It receives the signal reflected from a moving object, whose frequency is shifted by the Doppler effect. It is evaluated after amplification in an amplifier with AGC. Signals are applied to the input of a control amplifier whose output is connected through a further amplifier to a current amplitude detector, connected together with a nominal value generator to an integrator. Output of the latter is connected to the control input of the control amplifier, so that interference amplitude is adjusted to a maximum value over a correspondingly long time constant. Themaximum is such that threshold of limiting diodes in the signal path of amplifier is not exceeded.