We use the bumper switch before, so I won't spend a lot of time on it. It's a very simple sensor, which has only two values. One when it is pressed and zero when not pressed, we can use it in a program like this to stop the robot once it bumps into a wall. Or we can use it as a limit switch to stop the robot arm when it reaches the lower limit of its range of motion. You can see the limit switch here in my robot, and I use that command which is highlighted there to stop the arm from moving once it reaches the lower limit.