DC motor on an L293D (pins 5, 3 and 4), on an Arduino Uno

// A DC motor on an L293D: speed on pin 5, direction on pins 3 and 4.
//
//   L293D EN1,2 (pin 1)  -> Uno pin 5 (PWM: the speed)
//   L293D IN1 (pin 2)    -> Uno pin 3
//   L293D IN2 (pin 7)    -> Uno pin 4
//   L293D OUT1 and OUT2  -> the motor's two leads
//   L293D VSS (pin 16)   -> 5V, the chip's logic supply
//   L293D VS (pin 8)     -> the motor supply (the breadboard power module)
//   L293D GND pins       -> GND, shared with the Uno
//
// An Uno pin gives about 20 mA; a small motor wants several hundred. The
// L293D is an H-bridge: the Uno only tells it which way and how fast, and it
// switches the motor's own supply. IN1 HIGH and IN2 LOW turn the motor one
// way, the reverse turns it the other, and both the same stop it. The enable
// pin gates both, so analogWrite() on it sets the speed.
//
// Never run a motor from an Uno pin directly: the current spike when it starts
// can reset the board or damage the pin.

const int ENABLE = 5;
const int DIRA = 3;
const int DIRB = 4;

void drive(bool forward, int speed, const char *what) {
  digitalWrite(DIRA, forward ? HIGH : LOW);
  digitalWrite(DIRB, forward ? LOW : HIGH);
  analogWrite(ENABLE, speed);
  Serial.println(what);
}

void stopMotor() {
  analogWrite(ENABLE, 0);
  Serial.println("stop");
}

void setup() {
  pinMode(ENABLE, OUTPUT);
  pinMode(DIRA, OUTPUT);
  pinMode(DIRB, OUTPUT);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: DC motor on an L293D");
}

void loop() {
  drive(true, 255, "forward, full speed");
  delay(2000);
  stopMotor();
  delay(1000);
  drive(false, 255, "backward, full speed");
  delay(2000);
  stopMotor();
  delay(1000);
  drive(true, 140, "forward, slow");
  delay(2000);
  drive(true, 255, "forward, fast");
  delay(2000);
  stopMotor();
  delay(2000);
}