Build a line-following robot with Arduino

A two-wheeled robot that follows a black line on the floor, using two infrared sensors, a motor driver and about thirty lines of Arduino code.

Age 10 and up. Three sessions of about an hour. Builder level.

What you need

• Arduino Uno (or a compatible board) and its USB cable
• L298N motor driver module
• Two IR line sensor modules (TCRT5000 type)
• Two small geared DC motors (BO motors) with wheels
• One caster wheel, and a chassis: a ready-made kit, or stiff cardboard
• Battery pack of four AA cells (6 V) with a switch
• Jumper wires, and black electrical tape for the track

Step by step

1. Build the base

   Fix the two motors at the back of the chassis, one on each side, and the caster wheel at the front. The robot should sit level and roll straight when you push it. Leave room at the front for the two sensors.

2. Mount the sensors

   Fix the two IR sensors at the front, facing down, about 1 cm above the floor and about 2 cm apart, so the black line fits between them. Closer to the floor reads more reliably than further away.

3. Wire the motors to the driver

   Connect the left motor to OUT1 and OUT2 on the L298N, and the right motor to OUT3 and OUT4. Leave the jumpers on ENA and ENB, so both motors are always enabled.

4. Wire the driver and sensors to the Arduino

   Connect IN1, IN2, IN3 and IN4 on the driver to Arduino pins 5, 6, 9 and 10. Connect each sensor's VCC to 5V, GND to GND, and OUT to pin 2 (left) and pin 3 (right). Connect the battery pack to the driver's 12V and GND terminals, and join the driver's GND to the Arduino's GND: without a shared ground nothing works.

5. Upload the program

   The idea is simple: if both sensors see the white floor, drive forward; if the left sensor sees the line, turn left; if the right one does, turn right. Most sensor modules read LOW over white and HIGH over black; if yours is the other way round, swap LOW and HIGH in the program.

   const int L1 = 5, L2 = 6, R1 = 9, R2 = 10;
   const int LEFT = 2, RIGHT = 3;
   
   void drive(int l1, int l2, int r1, int r2) {
     digitalWrite(L1, l1); digitalWrite(L2, l2);
     digitalWrite(R1, r1); digitalWrite(R2, r2);
   }
   
   void setup() {
     const int motors[] = {L1, L2, R1, R2};
     for (int p : motors) pinMode(p, OUTPUT);
     pinMode(LEFT, INPUT);
     pinMode(RIGHT, INPUT);
   }
   
   void loop() {
     bool leftOnLine = digitalRead(LEFT) == HIGH;
     bool rightOnLine = digitalRead(RIGHT) == HIGH;
   
     if (!leftOnLine && !rightOnLine) drive(HIGH, LOW, HIGH, LOW);  // forward
     else if (leftOnLine)             drive(LOW, LOW, HIGH, LOW);   // turn left
     else if (rightOnLine)            drive(HIGH, LOW, LOW, LOW);   // turn right
   }

6. Tune the sensors

   Each sensor has a small screw (a potentiometer) and an LED. Hold the robot over the white floor and turn the screw until the LED just changes, then check it changes back over the tape. Do this for both sensors.

7. Make a track and test it

   Lay a loop of black tape on a light floor, with gentle curves to start. Put the robot on it with the line between the sensors and switch on. When it follows the gentle track, add a tighter curve and see where it fails.

Safety

• It runs on four AA batteries, which is safe to handle. Don't swap in lithium cells without an adult who knows how to charge and store them.
• Unplug the battery before changing any wiring.

When it doesn't work

• One wheel spins backwards: Swap that motor's two wires on the driver.
• It drives straight over the line: Tune the sensors again, lower them closer to the floor, or slow it down: pins 5 and 9 can take analogWrite, so analogWrite(5, 150) instead of digitalWrite(5, HIGH) runs that motor at about 60% speed.
• Nothing moves, but the Arduino's light is on: Check the battery is switched on and that the driver's GND is joined to the Arduino's GND.
• It wobbles from side to side: That's normal for this simple program. The next step is proportional control: turn gently when slightly off the line and hard when far off.

What it teaches

• Circuits and wiring
• Reading sensors
• If-then logic in code
• Testing and tuning a real machine

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