The rear bump switch is visible, as well as the back of one of the Dustbuster fans (below the chassis). The big fray of wires on the top is the (barely resolvable) stack of electronics. There are five printed circuit boards in the stack, with lots of stray wires wandering off to various I/O boards mounted elsewhere on the robot. Visible on the sonar turret is the motion detector, which provides a 1-bit output saying that there is or isn't motion in its view.