How to make an Arduino LED chaser with an array

An Arduino LED chaser lights a row of LEDs one after another, so the light seems to run along the row. Keep the pin numbers in an array, then let a for loop step through it: light one, wait, switch it off, move to the next. Four LEDs or forty, the code stays the same length.

A list of pins

int pins[] = {9, 10, 11, 12};
// pins[0] is 9, pins[1] is 10, pins[2] is 11, pins[3] is 12

Places in an array count from 0, so a four-item list runs from pins[0] to pins[3]. Wire each pin through a 220 Ω resistor to an LED, and each LED to GND.

A for loop visits each one

A for loop has three parts: where the count starts, when to keep going, and how it changes. This one runs its block for i = 0, 1, 2 and 3:

void setup() {
  for (int i = 0; i < 4; i = i + 1) {
    pinMode(pins[i], OUTPUT);       // all four pins in one line
  }
}

void loop() {
  for (int i = 0; i < 4; i = i + 1) {
    digitalWrite(pins[i], HIGH);
    delay(200);
    digitalWrite(pins[i], LOW);
  }
}

That's a chase: each step takes 200 ms, so a lap of four takes 0.8 s and the light runs round about 1.25 times a second. Halve the wait to 100 ms and it runs twice as fast.

Live experiment

A chase, then a bounce

Four LEDs on pins 9 to 12, kept in a list. A for loop counts i = 0, 1, 2, 3 and lights pins[i] in turn. Pick a chase or a bounce, 200, 100 or 50 ms a step, and tap the empty socket to add a fifth LED on pin 13.

Try it yourself: free lesson

“Many lights” is lesson 5 of 6 in Clockwork Cove, Embedded Systems World 2. Free with an account. Lessons unlock in order, so you’ll do the 11 before it first.

Bounce back like a scanner

To bounce, add a second loop that counts down. The two ends have just had their turn, so it runs from 2 to 1, not 3 to 0:

for (int i = 2; i >= 1; i = i - 1) {
  digitalWrite(pins[i], HIGH);
  delay(200);
  digitalWrite(pins[i], LOW);
}

Up then back gives 0, 1, 2, 3, 2, 1, and round again: six steps a lap, each the same length.

Add a light without rewriting

To add a fifth LED on pin 13, put 13 in the list and change i < 4 to i < 5 in each loop. Nothing else changes, because the loops visit whatever the list holds. Reading past the end, like pins[4] in a four-item list, isn't caught by the compiler: it reads whatever memory comes next, so keep the count and the list in step.

The chase uses delay(), which is fine while it's the only job. Give the board a button to read as well, and time each step with millis() instead.