If you’ve ever noticed how the sound of a fire truck’s siren has a higher pitch when it approaches you than when it moves away, this is due to the Doppler effect. In this case, when the vehicle is approaching you at a high speed, the number of sound wave that reach your ear over a period of time is increased. When it moves away, that number is then decreased. The actual wave frequency hasn’t changed, but your frame of reference has.
A Doppler sensor takes this somewhat interesting concept and turns it into useful information about a moving object’s speed relative to it. There are some pretty inexpensive ones to be found on eBay — like the CDM324 module — but they’re not more easily available than that yet. It seems these CDM324 modules do require a bit more circuitry for operation.
It’s becoming a rule that if you can find a cheap sensor, there’s a breakout board somewhere to make it easy to use. As outlined in his build writeup, Stephan Electronics has taken care of this extra circuitry in a tiny 3D-printed package. This module gives you a reflected frequency output to tell you a subject’s speed, as well as a voltage output proportional to the reflected signal’s strength. The result is very reliable sensing from this Doppler module despite somewhat lacking details in the OEM’s datasheet. See for yourself below.