PURPOSE: To prevent malfunction owing to radio interference by digitizing and demodulating by FM the Doppler signal included in the reflected waves from a moving body by a comparator, a one-shot multivibrator, a limiter and a rectifier circuit.

CONSTITUTION: The output of a microwave oscillator 1 is reflected by a moving body 6, and enters a mixer 5 via an antenna 4 and a circulator 3 by carrying Doppler signals, so that it is mixed with locally oscillated electric power, and the Doppler signal such as the one shown in figure (a) is obtained. From this signal, only the positive part is extracted by a comparator 10, and the signal is pulsed by a one- shot multivibrator 11. The amplitudes of the pulses are aligned by a limiter 12, and the pulses are rectified to the negative voltage output shown in figure (b) by a rectifier circuit 13. This signal is made into the AC signal as shown by figure (c) by a low-pass filter 14. This signal is applied via a Schmitt trigger circuit 8 to a relay driving circuit 9. Thereby, the output of the low-pass filter 14 is made only the DC component and only the moving body is surely detected despite interference of microwaves of close frequencies.