Wiring motors and sensors safely

// First test for an Arduino Uno + L298N-type driver + HC-SR04.
// Check your board's pin labels and datasheet: your pins may differ.
// First run: wheels OFF the ground.
const int ENA = 5, IN1 = 7, IN2 = 8;     // left motor
const int ENB = 6, IN3 = 9, IN4 = 10;    // right motor
const int TRIG = 2, ECHO = 3;            // ultrasonic sensor
const int IR_LEFT = 11, IR_RIGHT = 12;   // line sensors (used later)

void setMotor(int en, int a, int b, int speed) {   // speed from -255 to 255
  digitalWrite(a, speed > 0 ? HIGH : LOW);
  digitalWrite(b, speed < 0 ? HIGH : LOW);
  analogWrite(en, constrain(abs(speed), 0, 255));
}

void drive(int left, int right) {
  setMotor(ENA, IN1, IN2, left);
  setMotor(ENB, IN3, IN4, right);
}

long distanceCm() {
  digitalWrite(TRIG, LOW);  delayMicroseconds(2);
  digitalWrite(TRIG, HIGH); delayMicroseconds(10);
  digitalWrite(TRIG, LOW);
  long us = pulseIn(ECHO, HIGH, 30000);  // wait at most 30 ms for the echo
  if (us == 0) return -1;                // no echo: we do not know
  return us * 0.0343 / 2;                // there and back, so divide by 2
}

void setup() {
  int outputs[] = {ENA, IN1, IN2, ENB, IN3, IN4, TRIG};
  for (int p : outputs) pinMode(p, OUTPUT);
  pinMode(ECHO, INPUT);
  pinMode(IR_LEFT, INPUT);
  pinMode(IR_RIGHT, INPUT);
  Serial.begin(9600);
  drive(0, 0);
  delay(3000);                           // time to put the robot down and step back
}

void loop() {
  long d = distanceCm();
  Serial.println(d);
  if (d < 0 || d < 20) drive(0, 0);      // unsure or too close: stop
  else drive(150, 150);                  // gentle forward speed
  delay(50);
}