Two motors on an L293D motor shield, on an Arduino Uno

// Two DC motors on the L293D motor shield, with the AFMotor library.
//
// The shield's 74HC595 (pins 4, 7, 8 and 12) sets each motor's direction and
// its two L293Ds take the speed as PWM: M1 on pin 11, M3 on pin 6. The motors'
// own supply goes on EXT+ and EXT-, here a 9 V battery.

#include <AFMotor.h>

AF_DCMotor left(1);
AF_DCMotor right(3);

void setup() {
  right.setSpeed(180);
  right.run(BACKWARD);
  left.run(FORWARD);
}

void loop() {
  for (int speed = 0; speed <= 255; speed += 5) {
    left.setSpeed(speed);
    delay(20);
  }
  for (int speed = 255; speed >= 0; speed -= 5) {
    left.setSpeed(speed);
    delay(20);
  }
}