The HB100 Doppler Radar module is a moving object detector – these are commonly found in automatic doors at grocery stores and shopping malls and an interesting alternative to PIR based sensors. Unlike PIR, these sensors can be used to calculate speed of an object and whether the object is approaching or departing.  The HB100 uses a 10.525GHz microwave signal to detect movement. The sensor provides a signal (in millivolts) with a frequency period proportional to the velocity of the object. This small voltage will need amplification to be useable with any microcontroller.

The datasheet provides an example circuit and formula for calculating velocity. Large objects (vehicles) have been shown to be detectable well in excess of 50 meters away with sufficient amplification.