Abstract: A movement detector includes a transmitter (1) and a receiver (2) of electromagnetic, acoustic or light waves, and a frequency variation demodulator (3) connected to the receiver (2) and adapted to supply a signal which is progressively variable as a function of the movement detected. The movement detector further includes an analog-digital converter (5) which transforms the demodulated signal into a digital signal, a memory (6) connected to the converter (5), and an arrangement for controlling the loading of the memory. The memory (6) continuously makes available information corresponding to a memorized threshold signal. A comparator (8) continuously compares the memorized threshold signal with the signal given off by the demodulator (3) and emits a signal when the former is less than the latter.