Microwave Radar Based Human Body Movement Sensor - RCWL 0516 - Motion Sensor - 5-8 Meters This sensor module is ideal for human motion detection in a close vicinity of 5-8 meters. It is ideal for appliances and lights control in washrooms and other indoor environments. It can detect human movement in 360 degree angle with an effective working distance of 5 to 8 meter which is enough for small rooms. It provides direct output signal at 3.3V logic level which can be utilized to switch on some relay or pass on the signal to microcontroller for further actions. Highly accurate motion detector, can replace the PIR detectors in a very reliable manner. Grab one for your design!  ..

This is a pyrolectric sensor (PIR) module which developed for human body detection. A PIR detector combined with a fresnel lens are mounted on a compact size PCB together with an analog IC, and limited components to form the module. It detect the  motion by detecting the changes in the IR levels emitted from the object. It can be used as motion detector for security systems or robotics. It works from 3.3V to 5V DC and gives TTL output which can be directly given to microcontroller or to relay through a transistor. After switching ON the power supply to this module, it requires a settling time of approximately 60 seconds. Module dimensions are 2.5cm x 3.4cm. It uses BISS0001 Micropower PIR Motion detector IC. Datasheet of BISS0001 PIR Motion Detector Module Features • Detection range: 2-3 meters  • Settling time: 60 seconds  • Compact size (25 x 34 mm) • Supply Voltage: DC3V-5V • Current drain :< 50uA • Voltage Output: High/Low level signal: 0-3.3V • High sensitivity • Operation Temperature: -15C -70C • Infrared sensor: dual element, low noise, high sensitivity • Light sensor: CdS photocell A Good Tutorial on PIR Sensors :  http://learn.adafruit.com/pir-passive-infrared-proximity-motion-sensor/..