Obstacle-Avoiding Robot with HC-SR04 and Arduino

This is the project that makes people say “it’s alive”: the car drives around a room, stops before hitting things, looks left and right, and picks the open path. It builds directly on the 2WD robot car — same chassis, same drive code, plus one sensor and one servo.

How the HC-SR04 works

The HC-SR04 is sonar: pulse the TRIG pin for 10 µs and it emits an ultrasonic chirp at 40 kHz; the ECHO pin then goes HIGH until the reflection returns. Sound covers about 1 cm in 58 µs round-trip, so distance_cm = echo_time_µs / 58. Practical range is 2 cm to ~3–4 m, in a cone roughly 15° wide. Soft, angled, or very thin objects (curtains, chair legs) reflect poorly — that is physics, not a bug, and the code below treats “no echo” as open space for that reason.

Extra parts needed

Mounting and wiring

Mount the servo at the front centre of the chassis and fix the HC-SR04 to the servo horn facing forward at the 90° position — eyes level, not tilted at the floor.

Test the sensor alone

Wheels off the ground, upload a minimal sketch that prints readDistanceCm() (copy the function from the code below) every 200 ms and watch the Serial Monitor while moving a book toward and away from the sensor. You should see stable centimetre readings from ~3 cm to a couple of metres. Do not proceed until this works — debugging sonar and driving logic at the same time is misery.

The full roaming code

// Hobby Robots — obstacle-avoiding robot
// Drive pinout as per the robot car guide. Servo on D9, HC-SR04 on D11/D12.
#include <Servo.h>

const int ENA = 5, IN1 = 7, IN2 = 8;
const int ENB = 6, IN3 = 4, IN4 = 2;
const int TRIG = 12, ECHO = 11;
const int SERVO_PIN = 9;

const int SAFE_CM   = 25;    // closer than this = obstacle
const int CRUISE    = 170;

Servo neck;

void drive(int left, int right) {
  digitalWrite(IN1, left  >= 0 ? HIGH : LOW);
  digitalWrite(IN2, left  >= 0 ? LOW  : HIGH);
  analogWrite (ENA, min(abs(left), 255));
  digitalWrite(IN3, right >= 0 ? HIGH : LOW);
  digitalWrite(IN4, right >= 0 ? LOW  : HIGH);
  analogWrite (ENB, min(abs(right), 255));
}

long readDistanceCm() {
  digitalWrite(TRIG, LOW);  delayMicroseconds(2);
  digitalWrite(TRIG, HIGH); delayMicroseconds(10);
  digitalWrite(TRIG, LOW);
  long us = pulseIn(ECHO, HIGH, 25000UL);   // ~4 m timeout
  if (us == 0) return 400;                  // no echo: treat as clear
  return us / 58;                           // microseconds -> cm
}

long lookAt(int angle) {        // 0 = right, 90 = ahead, 180 = left
  neck.write(angle);
  delay(350);                   // let the servo arrive
  return readDistanceCm();
}

void setup() {
  pinMode(ENA, OUTPUT); pinMode(IN1, OUTPUT); pinMode(IN2, OUTPUT);
  pinMode(ENB, OUTPUT); pinMode(IN3, OUTPUT); pinMode(IN4, OUTPUT);
  pinMode(TRIG, OUTPUT); pinMode(ECHO, INPUT);
  neck.attach(SERVO_PIN);
  neck.write(90);
  delay(2000);
}

void loop() {
  if (readDistanceCm() > SAFE_CM) {
    drive(CRUISE, CRUISE);              // path is clear
    delay(60);
    return;
  }
  // Obstacle: stop, back off, look both ways, take the open side.
  drive(0, 0);              delay(150);
  drive(-150, -150);        delay(350);
  drive(0, 0);
  long right = lookAt(15);
  long left  = lookAt(165);
  neck.write(90);
  if (left > right) drive(-160, 160);   // rotate left
  else              drive(160, -160);   // rotate right
  delay(420);                           // ~90 degrees; tune this
  drive(0, 0);
}

Tuning and behaviour tweaks

• Stops too late: raise SAFE_CM, or lower CRUISE — stopping distance grows with speed.
• Turn angle wrong: the delay(420) spin is open-loop; tune it per surface (carpet needs more, tile less). Wheel encoders fix this properly.
• Twitchy near walls: average 3 readings and act on the median to reject the HC-SR04’s occasional ghost echo.
• Make it smarter: scan 5 angles instead of 2 and steer proportionally toward the most open direction.

Troubleshooting

Next steps