Build an Arduino line follower: eyes, knob, kill switch

To build an Arduino line follower with a speed knob and a kill switch, wire two downward-facing reflectance sensors either side of the line, a potentiometer for speed and a push-button for stop and go, then write one loop() with no delay() that reads all four inputs every pass and decides, in order: stopped, line on the left, line on the right, or straight on.

The wiring

• Eyes: two IR reflectance sensors on A0 (left) and A1 (right), reading about 937 over a pale floor and 250 over a black line
• Speed knob: a potentiometer across 5 V, its wiper on A2
• Kill switch: a push-button from pin 2 to GND, with INPUT_PULLUP
• Wheels: a motor driver taking PWM from pins 9 (left) and 10 (right)

Mount the eyes a few millimetres above the floor, a little more than the line's width apart, so that with the robot centred both see floor and the line runs between them. Print their readings over floor and line before writing any rules: the threshold has to sit between the two.

Plan the program

1. The toggle: flip stopped on the change from HIGH to LOW, and remember the reading in last.
2. The speed: map(analogRead(A2), 0, 1023, 0, 255).
3. The eyes: read both, every pass.
4. The decision, in order: stopped first, then each line rule, then straight.

The code

bool now = digitalRead(2);
if (now == LOW && last == HIGH) stopped = !stopped;
last = now;

int speed = map(analogRead(A2), 0, 1023, 0, 255);
int left = analogRead(A0);
int right = analogRead(A1);

if (stopped) {                        // the kill switch beats everything
  analogWrite(9, 0);     analogWrite(10, 0);
} else if (left < 512) {              // line under the left eye: swing left
  analogWrite(9, 0);     analogWrite(10, speed);
} else if (right < 512) {             // line under the right eye: swing right
  analogWrite(9, speed); analogWrite(10, 0);
} else {                              // centred: straight on
  analogWrite(9, speed); analogWrite(10, speed);
}

To swing left, stop the left wheel and run the right: the rover pivots round its stopped wheel and its nose swings back toward the line. Every running wheel uses speed, so half a turn of the knob gives half-speed straights and half-speed turns.

Live experiment

Drive the finished runner

This is the finished line runner on its course. Press KILL to set it off, turn the speed knob, and tick off the four checks: full speed with floor under both eyes, a swing left, a swing right at half speed, and a press of KILL that stops both wheels.

Try it yourself: free lesson

“Project: Line runner” is the project that ends Rover Ridge, Robotics World 1. Free with an account. Lessons unlock in order, so you’ll do the 6 before it first.

Test it like a checklist

• Floor under both eyes, knob full: both wheels at full.
• Left eye on the line: swings left.
• Right eye on the line, knob at half: swings right at half speed.
• Press KILL: both wheels stop, and stay stopped until the next press.

Once it runs, the three rules will zigzag down the track. Steering in proportion to the difference between the eyes is the upgrade that makes it glide.