In the first part of this series of articles, we looked at the philosophy behind the Lego Robotics Invention System, the features of the system and the power of the RCX block. In this second part, we give our attention to the manner in which the RCX block works with sensors, which are the eyes and ears of a robot. In addition, we reveal how you can build your own sensors.The RCX block, which is the key component of the Robotics Invention System, contains a complete microcontroller system. Up to three sensors can be connected to this block. These sensors enable the robot, to perceive ‘sensations’ from its environment. There are several types of ready-made sensors, including switches, reflective sensors, temperature pickups, rotation pickups and so on. Although the range of available sensors is fairly complete, Elektor readers will surely see the RCX interface as a challenge to develop their own, new sensors. As you will see, this is also possible. In addition, you will see that it is possible to connect more than one sensor to a single input.