2pcs hb100 Microwave Motion Sensor 10.525ghz Doppler Radar Detector

2pcs x HB100 Microwave Motion Sensor. If the radio waves encounter an object position is fixed, then the reflected wave frequency and transmitting frequency waves should be equal. If the object moves toward the launch, then the reflected waves will be compressed, that will increase the frequency of the reflected wave; otherwise the reflected wave's frequency decreases.

eBay