I was inspired by this topic and bought some cheap modules from ebay:
http://www.ebay.com/itm/5-8GHZ-Microwave-Radar-Sensor-6-9M-Smart-Switch-for-Home-Control-/401082379029?hash=item5d625f6b15:g:4BMAAOSwxp9W2Ser

http://www.ebay.com/itm/Microwave-Radar-Sensor-LED-Light-Control-Smart-Switch-Precise-for-Spherical-Lamp-/351673918969?hash=item51e16625f9:g:rYAAAOSwZ8ZW4MpE

Does anyone know how to hook them up? Was anyone able to make them work? The chinese seller is not really helpful in providing some more details.
I really like the idea of having a motion sensor behind a wall (read: out of sight), even if it means powering it from a wall socket.

I started playing with the first module listed in my previous post:

The module is really as easy to connect as a normal pir sensor. There are small text on the pcb. The top most connection (indicated with a 'o') is the sensor output (3,3v for a high and 0v for a low). The middle connection is ground and the bottom connection is Vcc (3,3 to 20v according to the ebay listing).

My setup was really simple, just a chinduino powered via usb. The sensor connected to +5v, ground and the output to analog pin A0 (I could have used a digital pin).

The first results are really promising. Some characteristics:

• The sensor is omnidirectional.
• Output is high (3,3V) for 30s when movement is detected
• New movement will restart the 30s timer
• 'low' power consumption. Triggered: 1.5mA; Idle: 1.4mA (measured @5v using a normal multimeter)
• The sensor doesn't react to temperature/light fluctuations (unlike most pir sensors)

Some range tests that triggered the output (note that this is probably not the maximum distance, just the stuff I tested):

• 0-4 meter distance (clear line of sight), moving my arm
• 5-8 meter distance (clear line of sight), walking around
• walking around at 5 meter distance with an indoor brick wall between me and the sensor

Stuff that I still need to test:

• Can the sensitivity be tweaked
• How to change the trigger timer to something else than 30s
• Duration test to see if the module is prone to false positives