You will find many uses for this Crystal-Locked Ultrasonic Movement Detector kit.   It is built around a matched pair of ceramic transducers which convert movement energy to electrical energy and vice versa.  The operating frequency of the pair is 40kHz.  Any movement in the area scanned by the transducer will be detected and a pulse produced.  In this kit the pulse turns on an LED.  Pads are provided to take this pulse to add-on circuits where it may be used to turn on buzzers, lights, etc.   A PCB-mounted switch can be used to switch between an automatic reset about 0.3 seconds after the detector has been triggered or to stay latched on.  The unit will reliably work from four meters to over 8 meters depending on the sensitivity setting and the direction of the movement.