How to start particular event for particular time using millis()?

Question

In my recent project, I want to run particular event for the particular time. For example, If I want to Turn ON my LED for 20 Second. And rest time LED must be off. I know using delay() it is possible but If we used delay() function then that time we can't do anything. So, I tried out millis() for that. I tried the basic code for this.

The code is below:

const int ledPin =  LED_BUILTIN;

int ledState = LOW;             
unsigned long previousMillis = 0;          
const long interval = 5000;           

void setup() 
{
  pinMode(ledPin, OUTPUT);
}

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

  if (currentMillis - previousMillis >= interval)
  {
    previousMillis = currentMillis;

    if (ledState == LOW) {
      ledState = HIGH;
    } else {
      ledState = LOW;
    }

    digitalWrite(ledPin, ledState);
  }
} 

But the output of this is like LED ON for 5 Second and then LED OFF for 5 Second. And this thing happens continuously.But I don't want this. I just want to turn ON LED for 5 Second when I want and then LED off forever. And I tried out many logics for that somehow can't able to get this thing. Someone suggest me some idea about this.

Answer 1

Use an extra boolean variable. Some use the previousMillis and make it zero to tell that the delay is not active, but please don't do that. Let previousMillis be the value of millis and use an extra boolean variable.

To start the millis delay, three things are needed: enable the delay, set previousMillis, and turn the led on.

To stop the millis delay, two things are needed: disable the delay, turn the led off. There is no need to do something with the previousMillis since the millis delay will be disabled.

const int ledPin =  LED_BUILTIN;

unsigned long previousMillis;
const unsigned long interval = 5000UL;
boolean enableDelay = false;

void setup()
{
  pinMode(ledPin, OUTPUT);
}

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

  // Trigger the delay, I use analogRead.
  if (analogRead(A0) > 500)
  {
    enableDelay = true;
    previousMillis = currentMillis;
    digitalWrite(ledpin, HIGH);
  }

  // Checking the millis delay should be done in the loop.
  // This code is not inside the code to start the delay, but
  // at the same level as other code in the loop.
  if (enableDelay)
  {
    if (currentMillis - previousMillis >= interval)
    {
      enableDelay = false;
      digitalWrite(ledPin, LOW);
    }
  }
}

There are libraries that can do this. The above code is the basic way to make a millis delay. I have changed the variable "interval" to a unsigned long, because the other two (currentMillis and previousMillis) are also unsigned long. That makes it more straightforward. The currentMillis and previousMillis must be unsigned long, but it is okay if "interval" is a long.

This way of a millis delay can be used with a finite state machine. If your sketch has many states, and one of those states is to keep a fan or a motor on for some time, then you can use it. The variable "enableDelay" is probably not needed in a finite state machine, because the "state" variable tells if that specific state (with the millis delay) is active.