Hi !,
I'm going to build simple ultrasonic obstacle avoiding robot .I know that it can be easily done if I bought motor control board,but I'm looking for a project that I can build motor control board too,then the only component that I want to buy is Ultrasonic sensor,other parts would make my self by assembling parts.I think you can understand what I'm looking for.I'm looking for a diagram that can detect sensor's signals and control the speed of the motors.
thank you!

How large is this robot? If it is really /really/ tiny, you might get away with using the PWM pins directly, or bit-banging other pins. Otherwise, you could use the proto shield, and solder an L298 on it or something.

EDIT: I was assuming this was an Arduino robot. If it isn't...

the only arduino part is ultrasonic module HC-SR04.the robot will be about A5 book size (half of A4 sheet) or little bit more

Which Arduino board are you using? (Uno or Due or something else?) Just curious.

No I'm not using any arduino board as motor control board. I'm going to make it my self

Nice So... You're using an Atmel chip of some sort? (I mean, for the main processing.) Either way, you're going need this:
http://www.mouser.com/Power/Power-Manag ... c0&FS=True