In their simplest form, short range radar sensors are excellent motion detectors. They provide very fine sensitivity for motion while providing a wide viewing angle (70-80˚) and far detection range (10's m). OmniPreSense recently released an API update for its OPS241 and OPS242 radar sensors which enables a simple interrupt pin with filtering for objects. An example use case is detecting people walking on a sidewalk but not the cars moving nearby.
The OPS241 and OPS242 offer a wide field of view (FOV) between 76-78˚ and a detection range of up to 10m for people and 25m for cars. The simple API provides complete speed and direction reporting over USB and UART interfaces for objects detected in the FOV. The latest v1.3.1 API update enables a single interrupt pin which triggers based upon filter settings for speed, signal magnitude, and direction. In the case of the example noted earlier, people walk around 1.4 m/s while a very slow moving car is 2.2 m/s (5 mph). By setting the filtering to trigger the interrupt when a motion is detected > 0.8 m/s and < 2 m/s, the sensor can filter out motion of the car from people on the sidewalk. Further filtering for signal magnitude and direction can be applied as well.
The interrupt pin on the OPS241 is 5V tolerant and best for Arduino solutions while the OPS242 interrupt signal is 3.3V tolerant and can be used on Raspberry Pi's and similar processor boards. The interrupt also has hysteresis built in so as not to trigger on glitches. A minimum of two consecutive reports meeting the filter settings must be seen before the trigger is set and at least two consecutive reports not meeting the filtering before the trigger goes away.
OmniPreSense provides an application note AN-15 Simple Motion Detection Interrupt with a full explanation of the operation.