This is an array of 8 sensors with controlled light source, returning you values of the sensor readings. Your program can then decide if you wish to follow line with it or do something else. Perfect addition for Mindstroms NXT kit.

Multi-Sensitivity Acceleration Sensor (ACCL-Nx-v3) is a 3 axis digital acceleration sensor designed for Lego mindstorms NXT. This sensor works at 4 different sensitivity levels of gravitiy.
The sensitivity can be chosen in your program.
This sensor can be used to measure static and dynamic acceleration in different axes. You can use this on your robot as tilt sensor or to track the velocity profile.

For ages 12 years + up and contains 149 pieces Add this expansion set to the Robotics Invention System to create creatures that can bite, sting, pinch, and pounce. Use flashing fiber optics to alarm intruders or lure unsuspecting prey. Chose from 149 new parts. Conquer 12 CD-ROM Challenges using hints and tips from the Extreme Creatures Constructopedia. Program your creature to work in a pair or a pack. This set includes: – 149 pieces – extreme creatures cd-rom software – extreme creatures constructopedia – 12 guided challenges – fiber optic strands. Requires Robots Invention System (RIS) not included.

Lego Robotics Mindstorms Set #9719
Build and program your own Lego robots! You can create thousands of robotic inventions. There are interactive training missions and challenges on the enclosed CD-ROM, the Lego Mindstorms website and your own imagination. Using your PC and the Lego Mindstorms CD-ROM software create a program that lets your invention think and act on it own. With a click of a mouse you can instantly download your program from your PC to your RCS using the infrared transmitter.
Works with Windows 95/98
requires a Pentium 90 MHz Computer
16 MB ram
At least 50 MB HD space
QT 2.1 included on the CD
Age 12+

Set contains over 727 LEGO pieces