Two LEDs, Two Speeds

Part of the Timing Without Delay section of Coddy's Arduino journey. Lesson 4 of 18.

With delay(), two LEDs blinking at different speeds is a puzzle: each wait holds up the other LED. With millis() it is easy, because nothing waits.

Give each job its own variables: its own last time and its own state. Then give each job its own if in loop(). Every loop, each job checks its own clock and acts only when its time has come:

unsigned long redPrevious = 0;
unsigned long greenPrevious = 0;

void loop() {
  unsigned long now = millis();
  if (now - redPrevious >= 500) {
    redPrevious = now;
    // switch the red LED
  }
  if (now - greenPrevious >= 200) {
    greenPrevious = now;
    // switch the green LED
  }
}

The two jobs never get in each other's way. You can add a third and a fourth the same way: one set of variables and one if each.

Never share one last-time variable between two jobs. The faster job resets it so often that the slower one never reaches its interval.

Challenge

Easy

The red LED on pin 13 already switches every 500 ms. Make the green LED on pin 12 switch every 200 ms at the same time:

add greenPrevious and greenState variables
add a second if in loop() for the green LED

Press Run and watch both blink, then Check.

Try it yourself

const int redLed = 13;
const int greenLed = 12;

unsigned long redPrevious = 0;
int redState = LOW;

// add the green LED's two variables here

void setup() {
  pinMode(redLed, OUTPUT);
  pinMode(greenLed, OUTPUT);
}

void loop() {
  unsigned long now = millis();

  // the red LED switches every 500 ms
  if (now - redPrevious >= 500) {
    redPrevious = now;
    if (redState == LOW) {
      redState = HIGH;
    } else {
      redState = LOW;
    }
    digitalWrite(redLed, redState);
  }

  // make the green LED switch every 200 ms here
}

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

All lessons in Timing Without Delay

1millis()

The millis() Clock Times in unsigned long Blink Without DelayTwo LEDs, Two SpeedsBlink While a Button Toggles A Light on a Timer

4Precise Timing

Measure With micros()When millis() Rolls Over Button Stopwatch

2State Machines

Remember the Mode Choosing With switch Named States With enum Traffic Light State Machine Cycle Modes With a Button

3Interrupts

A Press the Loop Misses FALLING, RISING and CHANGE Sharing With volatile Count Presses by Interrupt

Practice on your own: Arduino playground