The R3-B9 droid will have an obstacle-avoiding mode and also a wall-following mode. Therefore I am including three ultrasonic sensors HC-SR04 (available for 90 cents here) mounted at the bottom of the body, at the front and the two sides. This page has a very nice description of the sensor (but make sure to read the comment at the bottom by David, how to use the sensor in single-pin mode, and to use the “timeout” parameter in the Arduino code). I am using a recessed mount for the three ultrasonic sensors.
To have some acoustic output, I am adding a 2″ speaker, powered by a little 3W amp PAM8403 (available here for 99 cents), mounted behind a vent-like cover.
Astromech Droid R3-B9 