Blink Without Delay

Part of the Timing Without Delay section of Coddy's Arduino journey. Lesson 3 of 18.

delay() stops the whole sketch. While a blink waits its 500 ms, nothing else in loop() runs: a button pressed in that time is not seen until the wait is over.

The fix is to never wait. Let loop() run again and again, very fast, and each time compare the clock with the moment the LED last changed. Until 500 ms have passed, do nothing. Then change the LED and store the new moment:

const unsigned long interval = 500;
unsigned long previousMillis = 0;   // when the LED last changed

void loop() {
  unsigned long now = millis();
  if (now - previousMillis >= interval) {
    previousMillis = now;           // remember this moment
    // switch the LED here
  }
  // anything else here runs thousands of times a second
}

now - previousMillis is the time since the last change. To switch the LED, keep its state in a variable, int redState = LOW;, flip it with if/else and write it with digitalWrite(redLed, redState);.

Forget to update previousMillis and the condition stays true after the first 500 ms, so the LED changes on every loop and looks half lit.

Challenge

Medium

The yellow LED should follow the button, but it reacts late because the red LED's delay() calls stop the loop. Rewrite the red blink with millis():

keep the button part as it is
every 500 ms, switch the red LED on pin 13
no delay() anywhere

Press Run, hold the button and watch both LEDs, then Check.

Try it yourself

const int redLed = 13;
const int yellowLed = 12;
const int buttonPin = 2;

void setup() {
  pinMode(redLed, OUTPUT);
  pinMode(yellowLed, OUTPUT);
  pinMode(buttonPin, INPUT_PULLUP);
}

void loop() {
  // The yellow LED follows the button. Keep this part.
  if (digitalRead(buttonPin) == LOW) {
    digitalWrite(yellowLed, HIGH);
  } else {
    digitalWrite(yellowLed, LOW);
  }

  // The red LED blinks, but each delay() stops the lines above
  // for half a second. Rewrite this part with millis().
  digitalWrite(redLed, HIGH);
  delay(500);
  digitalWrite(redLed, LOW);
  delay(500);
}

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 longBlink Without DelayTwo LEDs, Two Speeds Blink 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