Edge Avoiding Robot using

Arduino Hands-On Programming and Robotics Course

If you want an Edge Avoiding Robot using only Arduino + L298N + HC-SR04 Ultrasonic Sensor, mount the ultrasonic sensor facing downward at the front edge of the robot.

Working

Normal floor distance: 3–10 cm (depending on mounting height).
When the robot reaches a table edge, the ultrasonic sensor sees a much larger distance (or no echo).
Robot moves backward and turns away from the edge.

Connections

HC-SR04

HC-SR04	Arduino
VCC	5V
GND	GND
TRIG	D9
ECHO	D10

L298N

L298N	Arduino
IN1	D2
IN2	D3
IN3	D4
IN4	D5

Keep ENA and ENB jumpers ON.

Arduino Code

#define TRIG 9
#define ECHO 10

#define IN1 2
#define IN2 3
#define IN3 4
#define IN4 5

long duration;
int distance;

void setup()
{
  pinMode(TRIG, OUTPUT);
  pinMode(ECHO, INPUT);

  pinMode(IN1, OUTPUT);
  pinMode(IN2, OUTPUT);
  pinMode(IN3, OUTPUT);
  pinMode(IN4, OUTPUT);

  Serial.begin(9600);
}

void loop()
{
  distance = getDistance();

  Serial.print("Distance: ");
  Serial.println(distance);

  // Floor detected
  if(distance < 15)
  {
    forward();
  }
  // Edge detected
  else
  {
    stopRobot();
    delay(200);

    backward();
    delay(600);

    stopRobot();
    delay(200);

    rightTurn();
    delay(600);

    stopRobot();
    delay(200);
  }
}

int getDistance()
{
  digitalWrite(TRIG, LOW);
  delayMicroseconds(2);

  digitalWrite(TRIG, HIGH);
  delayMicroseconds(10);

  digitalWrite(TRIG, LOW);

  duration = pulseIn(ECHO, HIGH, 30000);

  if(duration == 0)
    return 100;

  return duration * 0.034 / 2;
}

void forward()
{
  digitalWrite(IN1, HIGH);
  digitalWrite(IN2, LOW);

  digitalWrite(IN3, HIGH);
  digitalWrite(IN4, LOW);
}

void backward()
{
  digitalWrite(IN1, LOW);
  digitalWrite(IN2, HIGH);

  digitalWrite(IN3, LOW);
  digitalWrite(IN4, HIGH);
}

void rightTurn()
{
  digitalWrite(IN1, HIGH);
  digitalWrite(IN2, LOW);

  digitalWrite(IN3, LOW);
  digitalWrite(IN4, HIGH);
}

void stopRobot()
{
  digitalWrite(IN1, LOW);
  digitalWrite(IN2, LOW);

  digitalWrite(IN3, LOW);
  digitalWrite(IN4, LOW);
}

Important

Point the HC-SR04 downward toward the floor, not forward.
Measure the distance from the sensor to the floor and adjust this line if needed:

if(distance < 15)

For example:

Sensor height = 5 cm → use 8–10 cm threshold.
Sensor height = 10 cm → use 12–15 cm threshold.

This method works, but IR sensors are usually more reliable for edge-avoiding robots. Ultrasonic edge detection requires careful mounting and threshold tuning.