Here it is:
The small metallic cylinders at the top are the ultrasonic transducers. Each work on its own; it transmits a ping, and switches to listening mode.
If there is no obstacle, both wheels turn, and it moves forward. If one sensor is blocked, the opposite wheel is stopped. If both are blocked, then only the left wheel turns (default action).
All CMOS IC construction. NO CPU's at all...
Here is a shot from behind. The gearmotors for each wheel and the battery box are visible. Also the twisted red&black wires going to the piezo beeper can be seen. It chirps if an echo is picked up! Nice feature. The backplane is simply a piece of sheet metal, bent into an U shape. Everything screws onto this.
This thing is a showstopper! It is so simple, but the kids like it very much (of all ages)!
And, no it does not see things like pillows, and other soft things. It also does not see hard, shiny surfaces unless it is directly pointed at them; the ping just bounces off to somewhere else. It will however see humans; no matter how dressed-up, or naked they are! It also sees cylindrical objects, even if they are very thin.
The sensing distance is up to 30~40 cm, depending on the day...
Sorry, no schematics... This thing is sold commercially, so I can't supply anyone with circuit diagrams etc... But the idea is enough to build one! You can get resources from various books/magazines if you need to..