The SR04 robot was designed and built by David P. Anderson. It was David's fourth robot, hence the "04", a bit curious what the "SR" stands for (small robot?).
From David's description:
The design which has emerged is a small dual-differential drive platform with the geometry of an 11" circle. The drive wheels and tail caster sit on the perimeter of this circle, and thus it can rotate in its own space. This greatly simplifies software for maneuvering and collision avoidance.
CPUThe robot controller is a Motorola HC6811 microprocessor running in an M.I.T. 6.270 board. This card was developed for the introductory robotics course taught at M.I.T. It provides the HC6811 processor with 32k of battery-backed RAM, 32 eight-bit A/D channels, hardware 40khz IR , 6 one-amp H-Bridge (L293) motor driver channels, an LCD readout, and some digital I/O ports for timer and output compare functions useful to robot builder-types.