Multiple timed events with millis() in Arduino - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to get the current time in milliseconds.

Arduino

unsigned long currentTime = [1]();

Drag options to blanks, or click blank then click option'

Amillis

Bdelay

Cmicros

Dtime

Attempts:

3 left

💡 Hint

Common Mistakes

Using delay() instead of millis()

Using micros() which returns microseconds

The millis() function returns the number of milliseconds since the Arduino started running.

2fill in blank

medium

Complete the code to check if 1000 milliseconds have passed since last event.

Arduino

if (currentTime - previousTime [1] 1000) {

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A<

B<=

C==

D>=

Attempts:

3 left

💡 Hint

Common Mistakes

Using < instead of >=

Using == which may miss the event

We check if the difference is greater than or equal to 1000 ms to trigger the event.

3fill in blank

hard

Fix the error in updating the previous time after the event triggers.

Arduino

previousTime = [1];

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AcurrentTime - 1000

BcurrentTime

CpreviousTime

DcurrentTime + 1000

Attempts:

3 left

💡 Hint

Common Mistakes

Adding or subtracting 1000 to currentTime

Not updating previousTime at all

We update previousTime to the current time to reset the timer.

4fill in blank

hard

Fill both blanks to create a dictionary of events with their intervals.

Arduino

unsigned long intervals[] = [1], [2];

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A1000

B500

C2000

D3000

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same value twice

Using values that are too close or confusing

The array holds intervals of 500 ms and 2000 ms for two events.

5fill in blank

hard

Fill all three blanks to check and update multiple timed events.

Arduino

for (int i = 0; i < 2; i++) {
  if (currentTime - previousTimes[[1]] >= intervals[[2]]) {
    previousTimes[[3]] = currentTime;
    // trigger event
  }
}

Drag options to blanks, or click blank then click option'

Ai

Bi + 1

Attempts:

3 left

💡 Hint

Common Mistakes

Using i + 1 which causes out of bounds

Mixing different indices for the same event

Use the loop index i to access arrays for previous times and intervals.

Practice

1. What is the main advantage of using millis() for timing multiple events in Arduino instead of delay()?

easy

A. It resets the Arduino automatically.

B. It stops the program until the time passes.

C. It makes the program run slower.

D. It allows the program to run other tasks while waiting.

Solution

1. Step 1: Understand delay() behavior

   delay() pauses the whole program, stopping all other actions.

2. Step 2: Understand millis() advantage

   millis() returns elapsed time without stopping the program, so other tasks can run simultaneously.

3. Final Answer:

   It allows the program to run other tasks while waiting. -> Option D

4. Quick Check:

   millis() enables multitasking [OK]

Hint: Remember: delay() stops, millis() doesn't [OK]

Common Mistakes:

• Thinking millis() pauses the program
• Confusing delay() with non-blocking timing
• Believing millis() resets Arduino

2. Which of the following is the correct way to declare a variable to store the last time an event occurred using millis()?

easy

A. unsigned long lastEventTime = 0;

B. int lastEventTime = 0;

C. float lastEventTime = 0.0;

D. char lastEventTime = '0';

Solution

1. Step 1: Identify the correct data type for time

   Since millis() returns an unsigned long value, the variable must be unsigned long.

2. Step 2: Check variable initialization

   Initializing to 0 is correct to mark the start time.

3. Final Answer:

   unsigned long lastEventTime = 0; -> Option A

4. Quick Check:

   Use unsigned long for millis() times [OK]

Hint: Use unsigned long for time variables with millis() [OK]

Common Mistakes:

• Using int which can overflow quickly
• Using float which is not precise for time
• Using char which is for characters, not numbers

3. What will be the output of the following Arduino code snippet if millis() returns 5000 at the moment of checking?

unsigned long previousMillis = 3000;
unsigned long interval = 1500;

if (millis() - previousMillis >= interval) {
  Serial.println("Event triggered");
} else {
  Serial.println("Waiting");
}

medium

A. Waiting

B. No output

C. Event triggered

D. Compilation error

Solution

1. Step 1: Calculate elapsed time

   Elapsed time = 5000 (current millis) - 3000 (previousMillis) = 2000 ms.

2. Step 2: Compare elapsed time with interval

   Interval is 1500 ms. Since 2000 >= 1500, the condition is true, so "Event triggered" should print.

3. Final Answer:

   Event triggered -> Option C

4. Quick Check:

   Elapsed time 2000 >= 1500 triggers event [OK]

Hint: Subtract previousMillis from millis() to check elapsed time [OK]

Common Mistakes:

• Mixing up >= and > operators
• Forgetting to subtract previousMillis
• Assuming output without calculation

4. Identify the error in this code snippet that tries to blink two LEDs at different intervals using millis():

unsigned long previousMillis1 = 0;
unsigned long previousMillis2 = 0;
const long interval1 = 1000;
const long interval2 = 2000;

void loop() {
  if (millis() - previousMillis1 >= interval1) {
    digitalWrite(LED1, !digitalRead(LED1));
    previousMillis1 = millis();
  }
  if (millis() - previousMillis1 >= interval2) {
    digitalWrite(LED2, !digitalRead(LED2));
    previousMillis2 = millis();
  }
}

medium

A. LED pins are not defined.

B. Second if condition uses previousMillis1 instead of previousMillis2.

C. Intervals should be unsigned long, not long.

D. digitalWrite cannot toggle LEDs.

Solution

1. Step 1: Check timing variables in conditions

   The second if condition incorrectly uses previousMillis1 instead of previousMillis2, causing wrong timing for LED2.

2. Step 2: Understand impact of error

   This mistake means LED2 timing depends on LED1 timing, so LED2 won't blink at its own interval.

3. Final Answer:

   Second if condition uses previousMillis1 instead of previousMillis2. -> Option B

4. Quick Check:

   Each event needs its own previousMillis variable [OK]

Hint: Use separate previousMillis for each timed event [OK]

Common Mistakes:

• Reusing the same previousMillis variable for multiple events
• Not updating previousMillis after event
• Confusing interval variables

5. You want to control three LEDs blinking at 500ms, 1000ms, and 1500ms intervals respectively without using delay(). Which approach correctly manages all three timed events using millis()?

hard

A. Use three separate previousMillis variables and check each with its own interval inside loop().

B. Use one previousMillis variable and reset it after each LED toggles.

C. Use delay(500) and toggle all LEDs together.

D. Use millis() only once and toggle LEDs based on dividing millis() by intervals.

Solution

1. Step 1: Understand independent timing needs

   Each LED needs its own timer to blink independently at different intervals.

2. Step 2: Evaluate options

   Use three separate previousMillis variables and check each with its own interval inside loop(), allowing independent timing without blocking.

3. Final Answer:

   Use three separate previousMillis variables and check each with its own interval inside loop(). -> Option A

4. Quick Check:

   Separate timers for each event [OK]

Hint: Assign each event its own timer variable for independent control [OK]

Common Mistakes:

• Using one timer for all events causing sync issues
• Using delay which blocks other events
• Trying to calculate toggles from one millis() value without state