The UHF motion detector operates on the Doppler radar principle. Ql is an oscillator that creates a radiated signal. An
object in the radiated field reflects some of this energy back to the detector. If the object is moving, the reflected signal will have a different frequency because of Doppler shift. Ql is an oscillator coupled to a small (8-cm) antenna. This antenna also receives the reflected signal. Dl acts as a mixer and produces a beat note of frequency that is equal to the difference in reflected and radiated signal. Q2 amplifies this signal and couples it to comparator/detector IC1A and IC1B. The output can continue on to an alarm, relay, lamp, etc.