Doppler Radar Motion Sensors - anyone used one

There seem to be a lot of devices like

http://www.ebay.com/itm/Microwave-Doppler-Velocity-Radar-Sensor-Wireless-Module-Move-Motion-Speed-Sensor-/370763837962?pt=LH_DefaultDomain_0&hash=item56533f320a

on eBay. ( Search for Doppler Radar sensor ). Prices in the 7-12 USD range.

Anyone used one of these gadgets ?

Those are the radar modules without comparators. It is useless.... until you build a radar detection circuit with comparators.

You should use the radar modules with a sensitivity pot on the back side. Those have a digital output if motion is detected.

http://www.parallax.com/tabid/768/ProductID/606/Default.aspx