A knob drives a motor both ways through an L293D on an Arduino Uno

// A knob drives a motor both ways through an L293D.
//
//   L293D 1A -> pin 10   2A -> pin 9   1,2EN -> pin 11
//   L293D VCC1 -> 5V   VCC2 -> the motor supply   GND -> GND
//   1Y and 2Y -> the motor   the knob's wiper -> A0
//
// The middle of the knob, 512, is stop. Further from it is faster: the
// distance becomes the PWM on the enable pin. Which side of the middle sets
// the direction: 1A HIGH and 2A LOW one way, the other way round the other.

const int IN1 = 10;
const int IN2 = 9;
const int ENABLE = 11;

void drive(bool forward, int speed) {
  digitalWrite(IN1, forward ? HIGH : LOW);
  digitalWrite(IN2, forward ? LOW : HIGH);
  analogWrite(ENABLE, constrain(speed, 0, 255));
}

void setup() {
  pinMode(IN1, OUTPUT);
  pinMode(IN2, OUTPUT);
  pinMode(ENABLE, OUTPUT);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: turn the knob away from the middle");
}

void loop() {
  int knob = analogRead(A0);
  bool forward = knob > 512;
  int speed = map(abs(knob - 512), 0, 512, 0, 255);
  drive(forward, speed);
  delay(50);
}