no microcontroller use L293D
I'm not sure what You ask, but i will try to guess. Robot that billhowl
linked You to does not use microcontroller. L293D is not uC - it is a motor driver, you either use that or use H-bridge consisting of at least 4 transistors.
single transistor used.
That won't happen
you can make H-bridge controller is smooth
Smooth H-bridge driving (or have You meant smooth motor control) requires PWM which in turn requires uC or something like 555 Timer in astable mode and a lot (and I mean A LOT) of hand tuning.
,but how do other
Sorry, do what now?
NOTE 1: You did not tell us what do You want Your robot to do.
NOTE 2: You would get more replies if Your sentences were constructed better (with more effort say).