Doppler Radar
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RCWL0516 Doppler Radar sensor (click to enlarge)

Doppler Radar boards with a digital output (on-off) generally sell for a few dollars from China. Unlike the Doppler Radar guns used by police to catch speeding cars, these just detect any motion, often in a detection area of 360° around the sensor (i.e., in any direction).

The following text is taken from a board sold by Smart Prototyping:

Description:#

This little board is a Doppler radar microwave motion sensor module. It detects motion in a 360-degree directional range and can serve as an alternative to a PIR motion sensor. Use it to pick up any moving objects, animals, or people. It can be hooked up to an output to create a DIY security system, program to wake up a device or create a wicked Halloween decoration (tutorial below!).

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How it works#

Doppler radar measures the Doppler effect, or an increase or decrease in the frequency of sound, light, or other waves as the source and observer move toward or away from each other.

Microwaves are electromagnetic radiations that are sent by a transmitter. They get reflected off of objects and based on the object's velocity, the frequency of the waves will be altered. The receiver will then detect the reflected waves and calculate the difference in the transmitted and received waves’ frequency. This difference is what is used for the board to sense motion.