The basic theory behind the ParKontroller is the Sound Navigation and Ranging (SONAR) technique that is used for finding the distance and direction of a remote object underwater by transmitting sound waves and detecting reflections from it.
http://instruct1.cit.cornell.edu/courses/ee476/FinalProjects/s2007/jjl49_mar97/jjl49_mar97/index.htm
An ultrasonic, or sonar, range finder is a common sensor in robotic systems and industrial environments. Even home and automotive uses are possible. A novel sensor design consists of a WC, a few peripheral components, and a pair of ultrasonic transducers.
http://www.edn.com/article/CA45965.html?spacedesc=designideas&industryid=44217
Lasers can be applied to a variety of important distance-measurement tasks. This chapter describes three such methods.
http://cord.org/cm/leot/Module6/module6.htm
In this circuit the TSOP4830 is used as a sensitive infrared detector that does not require shielding from room lighting. The detector will work under almost any lighting condition from complete darkness to full sunlight.
http://home.cogeco.ca/~rpaisley4/IrProximity.html
This range meter uses the ultrasonic of about 40 KHz. About 10-m distance can be measured by adjusting the resistance value.
http://www.hobby-elec.org/e_srm.htm
Ultrasonic Range Meter with PIC16F873.
http://www.hobby-elec.org/e_pic6_6.htm
This range meter uses the ultrasonic of about 40 kHz and uses PIC microcontroller.
http://www.hobby-elec.org/e_psrm.htm