Obstacle-avoiding robot

Learn

What you’ll build

A robot car that drives forward, stops when something is ahead, looks for the clearest way and turns that way. It’s a complete sense–think–act robot.

Parts

Part	Qty	Notes
2WD robot car chassis with 2 TT gear motors and wheels	1	Includes a caster
Arduino UNO (or compatible)	1
L298N or TB6612FNG motor driver	1
6 × AA battery holder with switch	1	About 9 V with alkalines
Jumper wires (male–male and male–female)	~20
HC-SR04 ultrasonic sensor	1	Front mounted
SG90 servo (optional)	1	To turn the sensor and “look” left and right

Wiring

Motors as in the table below; HC-SR04: VCC → 5V, GND → GND, TRIG → 3, ECHO → 2. Optional servo signal → pin 11, powered from 5V (or a separate 5 V supply).

Driver pin	Arduino pin	Purpose
ENA	5 (PWM)	Left motor speed
IN1 / IN2	7 / 8	Left motor direction
ENB	6 (PWM)	Right motor speed
IN3 / IN4	9 / 10	Right motor direction
12V (VM)	Battery + (through the switch)	Motor power
GND	Battery − and Arduino GND	Shared ground
5V	Arduino VIN is fed from the battery instead	Leave the driver’s 5V output unused, or check your board’s jumper

How it decides

1. Measure the distance ahead.
2. If it’s more than 25 cm, drive forward.
3. Otherwise stop, reverse briefly, then look left and right (turning the servo, or turning the robot) and pick the side with more space.
4. Repeat.

Tips

• Mount the sensor level and at the very front; a low angle makes it see the floor.
• Narrow obstacles (chair legs) can slip past one sensor: a servo sweep or two angled sensors helps.
• Reverse a little before turning, so the robot doesn’t clip the obstacle while spinning.

Examples

The full program (with an optional servo)

obstacle-avoider.ino

#include <Servo.h>
const int ENA = 5, IN1 = 7, IN2 = 8, ENB = 6, IN3 = 9, IN4 = 10;

void setMotor(int en, int a, int b, int speed) {   // speed -255..255
  digitalWrite(a, speed > 0);
  digitalWrite(b, speed < 0);
  analogWrite(en, min(abs(speed), 255));
}

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

void setupMotors() {
  int pins[] = {ENA, IN1, IN2, ENB, IN3, IN4};
  for (int p : pins) pinMode(p, OUTPUT);
}

const int TRIG = 3, ECHO = 2;
Servo neck;

float readCm() {
  digitalWrite(TRIG, LOW);  delayMicroseconds(2);
  digitalWrite(TRIG, HIGH); delayMicroseconds(10);
  digitalWrite(TRIG, LOW);
  long us = pulseIn(ECHO, HIGH, 30000);
  return us == 0 ? 400 : us * 0.0343 / 2;
}

float lookAt(int angle) {
  neck.write(angle);
  delay(300);          // let the servo arrive
  return readCm();
}

void setup() {
  setupMotors();
  pinMode(TRIG, OUTPUT); pinMode(ECHO, INPUT);
  neck.attach(11);
  neck.write(90);      // look straight ahead
  delay(1000);
}

void loop() {
  if (readCm() > 25) {
    drive(170, 170);
    return;
  }
  drive(0, 0);
  drive(-150, -150); delay(300);   // back off
  drive(0, 0);
  float left = lookAt(160), right = lookAt(20);
  neck.write(90);
  if (left > right) drive(-160, 160);   // spin left
  else drive(160, -160);                // spin right
  delay(350);
}

No servo? Delete the servo lines and, instead of looking, always turn the same way or alternate directions.

Practice

Try it in the simulator

The virtual robot has the same front distance sensor, plus left and right distances.

Obstacle avoider

▶ Run↺ Reset

Your code (JavaScript)function loop() { if (distance() > 25) { motors(60, 60); } else if (distanceLeft() > distanceRight()) { motors(-40, 40); } else { motors(40, -40); } }Load starter code

Robot commands

• motors(left, right): wheel power, −100 to 100
• lineLeft(), lineRight(): true over the black line
• distance(): cm to the nearest thing ahead (max 200)
• distanceLeft(), distanceRight(): cm to the sides
• linePosition(): where the line is, −1 (left) to 1 (right), or null
• millis(): milliseconds since Run
• print(value): show a message below
• setup() runs once; loop() runs 50 times a second

Check the sensor logic in the virtual Arduino

Check your sensor logic before building: press Start simulator, replace diagram.json, paste the sketch and press ▶. Drag the sensor’s distance slider below 25 cm and the LED lights.

diagram.json

{
  "version": 1,
  "author": "RoboLearn",
  "editor": "wokwi",
  "parts": [
    {
      "type": "wokwi-arduino-uno",
      "id": "uno",
      "top": 0,
      "left": 0,
      "attrs": {}
    },
    {
      "type": "wokwi-hc-sr04",
      "id": "sonar",
      "top": -110,
      "left": 90,
      "attrs": {
        "distance": "60"
      }
    }
  ],
  "connections": [
    [
      "sonar:VCC",
      "uno:5V",
      "red",
      []
    ],
    [
      "sonar:GND",
      "uno:GND.1",
      "black",
      []
    ],
    [
      "sonar:TRIG",
      "uno:3",
      "blue",
      []
    ],
    [
      "sonar:ECHO",
      "uno:2",
      "green",
      []
    ]
  ],
  "dependencies": {}
}

sketch.ino

const int TRIG = 3, ECHO = 2;
void setup() { pinMode(TRIG, OUTPUT); pinMode(ECHO, INPUT); pinMode(LED_BUILTIN, OUTPUT); Serial.begin(9600); }
void loop() {
  digitalWrite(TRIG, LOW); delayMicroseconds(2);
  digitalWrite(TRIG, HIGH); delayMicroseconds(10); digitalWrite(TRIG, LOW);
  long us = pulseIn(ECHO, HIGH, 30000);
  float cm = us == 0 ? 400 : us * 0.0343 / 2;
  Serial.println(cm);
  digitalWrite(LED_BUILTIN, cm < 25 ? HIGH : LOW);   // LED = "would stop here"
  delay(100);
}

Distance check before wiring the carOpen full screen ↗

Start simulator

Runs on Wokwi, free, no account needed.

Never get stuckEasy

In the simulator, find a starting code change that stops the robot getting trapped in corners. Reverse for a moment before turning, or turn for longer.

Speed by distanceMedium

Make the robot slow down smoothly as it approaches an obstacle (fast when far, slow when near). const s = Math.min(70, Math.max(20, distance())); if (distance() > 20) motors(s, s); else motors(-40, 40);

ExplorerHard

Make the robot visit all four corners of the obstacle room within 90 seconds.

Quick check

1. Why reverse briefly before turning?

To avoid clipping the obstacle while spinningTo charge the batteryIt looks coolTo reset the Arduino

2. What does the optional servo do?

Drives the wheelsTurns the sensor to look left and rightMeasures speedBeeps

3. The sensor keeps seeing the floor. Fix?

Mount it level and higherUse more batteriesPaint the floorLower the threshold to 1 cm

Check my answers