Hi...It is a Micro-wave Doppler effect sensor. The principle is simple : it sends microwave and does the difference between the sended wave frequency and the response wave frequency.
The difference of frequency is proportionnal to the speed of the object it detects.
But, as you might expect it, I have a problem : I've spent some time working on the output signal, it's a sine signal which amplitude is around 10 to 100 mV depending on the size and the distance of the object.
My problem is that I can't find a SIMPLE solution to amplify this signal in order to use it with an Arduino. I've tried several Amp Op such as LM381 or TL082 but with no luck : the LM381 doesn't work with AC signals and the TL082 seems to not detect the input signal