Arduino sleep modes - Mini Project: Build & Apply

Arduino Sleep Modes

📖 Scenario: You have a small Arduino project that runs on battery power. To save battery, you want the Arduino to sleep when it is not doing anything and wake up when a button is pressed.

🎯 Goal: Build a simple Arduino program that puts the board into sleep mode and wakes it up using a button press.

📋 What You'll Learn

Use the avr/sleep.h library to control sleep modes

Set up a button on pin 2 to wake the Arduino from sleep

Put the Arduino into sleep mode inside the loop()

Print a message to the Serial Monitor when the Arduino wakes up

💡 Why This Matters

🌍 Real World

Battery-powered devices like remote sensors or wearable gadgets use sleep modes to save power and extend battery life.

💼 Career

Understanding sleep modes is important for embedded systems engineers working on low-power IoT devices and hardware optimization.

1

Setup button input and Serial communication

Write code to set pin 2 as an input with an internal pull-up resistor. Also, start Serial communication at 9600 baud in the setup() function.

Arduino

void setup() {
  // Your code here
}

void loop() {
  // Your code here
}

Hint

Use pinMode(2, INPUT_PULLUP); to set the button pin. Use Serial.begin(9600); to start serial communication.

2

Configure sleep mode and interrupt

Add code to set the sleep mode to SLEEP_MODE_PWR_DOWN and attach an interrupt to pin 2 that will wake the Arduino when the button is pressed.

Arduino

#include 

void setup() {
  pinMode(2, INPUT_PULLUP);
  Serial.begin(9600);
  // Your code here
}

void loop() {
  // Your code here
}

Hint

Use set_sleep_mode(SLEEP_MODE_PWR_DOWN); to set the sleep mode. Use attachInterrupt(digitalPinToInterrupt(2), wakeUp, LOW); to attach the interrupt.

3

Put Arduino to sleep inside loop

Inside the loop(), add code to put the Arduino to sleep using sleep_enable() and sleep_cpu(). Make sure to disable sleep after waking up.

Arduino

#include 

void wakeUp() {
  // This function will be called when interrupt occurs
}

void setup() {
  pinMode(2, INPUT_PULLUP);
  Serial.begin(9600);
  set_sleep_mode(SLEEP_MODE_PWR_DOWN);
  attachInterrupt(digitalPinToInterrupt(2), wakeUp, LOW);
}

void loop() {
  // Add sleep code here
}

Hint

Use sleep_enable(); before sleep_cpu(); and sleep_disable(); after waking up. Print messages before and after sleep.

4

Test and observe output

Upload the complete program and observe the Serial Monitor. The Arduino should print Going to sleep..., then sleep until the button on pin 2 is pressed, then print Woke up!. Write the exact output you see after pressing the button once.

Arduino

#include 

void wakeUp() {
  // This function will be called when interrupt occurs
}

void setup() {
  pinMode(2, INPUT_PULLUP);
  Serial.begin(9600);
  set_sleep_mode(SLEEP_MODE_PWR_DOWN);
  attachInterrupt(digitalPinToInterrupt(2), wakeUp, LOW);
}

void loop() {
  Serial.println("Going to sleep...");
  delay(100); // Give time for message to send
  sleep_enable();
  sleep_cpu();
  sleep_disable();
  Serial.println("Woke up!");
  delay(1000); // Wait before sleeping again
}

// Observe output after pressing the button once

Hint

Open the Serial Monitor at 9600 baud. Press the button connected to pin 2 once. You should see the messages printed before and after sleep.

Practice

1. Which Arduino function is used to set the type of sleep mode before entering sleep?

easy

A. set_sleep_mode()

B. sleep_cpu()

C. attachInterrupt()

D. delay()

Solution

1. Step 1: Understand sleep mode setup

   The function set_sleep_mode() is used to choose which sleep mode the Arduino will enter.

2. Step 2: Differentiate from sleep execution

   The function sleep_cpu() actually puts the CPU to sleep, but does not set the mode.

3. Final Answer:

   set_sleep_mode() -> Option A

4. Quick Check:

   Set mode = set_sleep_mode() [OK]

Hint: Set mode first with set_sleep_mode() before sleeping [OK]

Common Mistakes:

• Confusing set_sleep_mode() with sleep_cpu()
• Thinking attachInterrupt() sets sleep mode
• Using delay() to enter sleep

2. Which of the following is the correct syntax to put the Arduino into sleep mode after setting it?

easy

A. sleep_mode();

B. sleep_cpu();

C. sleep();

D. enter_sleep();

Solution

1. Step 1: Identify the sleep function

   The function sleep_cpu() is the correct call to put the CPU into sleep mode after setting it.

2. Step 2: Check other options

   sleep_mode() is not a valid function, nor are sleep() or enter_sleep().

3. Final Answer:

   sleep_cpu(); -> Option B

4. Quick Check:

   Sleep call = sleep_cpu() [OK]

Hint: Use sleep_cpu() to enter sleep after setting mode [OK]

Common Mistakes:

• Using sleep_mode() instead of sleep_cpu()
• Trying to call sleep() which doesn't exist
• Confusing function names

3. What will the following Arduino code do?

set_sleep_mode(SLEEP_MODE_PWR_DOWN);
sleep_enable();
sleep_cpu();
// CPU sleeps here
sleep_disable();

medium

A. The CPU enters power-down sleep mode and stays asleep until an interrupt wakes it

B. The CPU enters power-down sleep mode and wakes immediately

C. The CPU does not sleep because sleep_enable() is missing

D. The CPU enters idle mode instead of power-down

Solution

1. Step 1: Analyze sleep mode setting

   The code sets the sleep mode to power-down with set_sleep_mode(SLEEP_MODE_PWR_DOWN);.

2. Step 2: Understand sleep flow

   Calling sleep_enable() then sleep_cpu() puts the CPU to sleep in power-down mode. It stays asleep until an interrupt or reset wakes it. After waking, sleep_disable() disables sleep.

3. Final Answer:

   The CPU enters power-down sleep mode and stays asleep until an interrupt wakes it -> Option A

4. Quick Check:

   Power-down sleep = stays asleep until interrupt [OK]

Hint: Power-down sleep waits for interrupt to wake CPU [OK]

Common Mistakes:

• Assuming CPU wakes immediately without interrupt
• Forgetting sleep_enable() disables sleep
• Confusing idle mode with power-down

4. Identify the error in this Arduino sleep code snippet:

set_sleep_mode(SLEEP_MODE_IDLE);
sleep_cpu();
sleep_enable();

medium

A. set_sleep_mode() is missing a parameter

B. No error, code is correct

C. sleep_cpu() should be replaced with sleep_mode()

D. sleep_enable() must be called before sleep_cpu()

Solution

1. Step 1: Check function order

   The function sleep_enable() must be called before sleep_cpu() to enable sleep mode properly.

2. Step 2: Verify other parts

   set_sleep_mode() is correctly called with a valid mode. sleep_cpu() is the correct function to enter sleep.

3. Final Answer:

   sleep_enable() must be called before sleep_cpu() -> Option D

4. Quick Check:

   Enable sleep before sleeping [OK]

Hint: Call sleep_enable() before sleep_cpu() to avoid errors [OK]

Common Mistakes:

• Calling sleep_cpu() before sleep_enable()
• Confusing sleep_cpu() with sleep_mode()
• Missing parameters in set_sleep_mode()

5. You want to save maximum power on your Arduino but still wake it up using a button press on pin 2. Which sleep mode and wake method should you use?

hard

A. Use SLEEP_MODE_IDLE and poll the button in loop()

B. Use SLEEP_MODE_STANDBY and wake with delay()

C. Use SLEEP_MODE_PWR_DOWN and attach an external interrupt on pin 2

D. Use SLEEP_MODE_ADC and check button state with analogRead()

Solution

1. Step 1: Choose lowest power sleep mode

   SLEEP_MODE_PWR_DOWN uses the least power by shutting down most parts of the chip.

2. Step 2: Select wake method

   To wake from power-down mode, an external interrupt on pin 2 (using attachInterrupt) is needed because polling or delay() won't wake the CPU.

3. Final Answer:

   Use SLEEP_MODE_PWR_DOWN and attach an external interrupt on pin 2 -> Option C

4. Quick Check:

   Lowest power + external interrupt wake = Use SLEEP_MODE_PWR_DOWN and attach an external interrupt on pin 2 [OK]

Hint: Lowest power sleep needs external interrupt wake [OK]

Common Mistakes:

• Using idle mode which uses more power
• Trying to wake with delay() which doesn't work
• Polling button instead of using interrupt