LowPower library usage in Arduino - Step-by-Step Execution

Concept Flow - LowPower library usage

Start program

↓

Setup hardware

↓

Call LowPower.sleep()

↓

Microcontroller sleeps

↓

Wake up by interrupt or timer

↓

Resume normal operation

↓

Repeat or end

The program sets up, then calls LowPower.sleep() to put the microcontroller to sleep. It wakes up on an interrupt or timer, then continues running.

Execution Sample

Arduino

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

void loop() {
  digitalWrite(LED_BUILTIN, HIGH);
  delay(1000);
  LowPower.sleep();
  digitalWrite(LED_BUILTIN, LOW);
  delay(1000);
}

This code turns on the LED, waits 1 second, sleeps to save power, then turns off the LED and waits 1 second.

Execution Table

Step	Action	State Before	State After	Output/Effect
1	Setup pinMode for LED	LED pin undefined	LED pin set as OUTPUT	No visible output
2	Turn LED ON	LED OFF	LED ON	LED lights up
3	Delay 1000 ms	LED ON	LED ON	LED stays ON for 1 second
4	Call LowPower.sleep()	LED ON	Microcontroller sleeps	CPU stops, LED stays ON
5	Wake up from sleep	Microcontroller sleeping	Microcontroller awake	Program continues
6	Turn LED OFF	LED ON	LED OFF	LED turns off
7	Delay 1000 ms	LED OFF	LED OFF	LED stays OFF for 1 second
8	Loop repeats	LED OFF	LED ON	LED lights up again
9	Repeat steps 3-7	Cycle continues	Cycle continues	LED blinks with sleep in between

💡 Loop runs indefinitely; LowPower.sleep() pauses CPU until wake-up event.

Variable Tracker

Variable	Start	After Step 2	After Step 4	After Step 6	After Step 7	After Step 9
LED state	OFF	ON	ON (sleeping)	OFF	OFF	ON

Key Moments - 3 Insights

Why does the LED stay ON during LowPower.sleep()?

Because LowPower.sleep() stops the CPU but does not turn off the LED pin output. The LED remains ON until the program changes it (see steps 4 and 6 in execution_table).

What wakes the microcontroller from LowPower.sleep()?

An interrupt or timer event wakes the microcontroller. This is why after step 4 (sleep), step 5 shows the microcontroller waking up and continuing execution.

Does LowPower.sleep() stop the entire program?

No, it only pauses the CPU to save power. The program resumes right after the sleep call when a wake-up event occurs (see step 5 and 6).

Visual Quiz - 3 Questions

Look at the execution_table, what is the LED state immediately after calling LowPower.sleep()?

A Blinking

B OFF

C ON

D Undefined

💡 Hint

Check Step 4 and Step 5 in the execution_table where LED state is shown before and after sleep.

At which step does the microcontroller wake up from sleep?

A Step 5

B Step 4

C Step 3

D Step 6

💡 Hint

Look for the step where the state changes from sleeping to awake in the execution_table.

If you remove the LowPower.sleep() call, what changes in the execution_table?

A The LED never turns ON

B The microcontroller never sleeps, so no sleep steps appear

C The LED stays ON forever

D The program crashes

💡 Hint

Refer to the execution_table steps involving sleep (Step 4 and 5) and consider what happens if those are missing.

Concept Snapshot

LowPower.sleep() pauses the microcontroller to save power.
The CPU stops but pins keep their state.
Wake-up happens via interrupts or timers.
Use in loop to save energy between tasks.
Setup pins before sleep; resume after wake.
No program reset, just pause and resume.

Full Transcript

This example shows how to use the LowPower library in Arduino to save energy. The program sets the LED pin as output, then in the loop it turns the LED on, waits one second, calls LowPower.sleep() to pause the CPU, then turns the LED off and waits another second. The microcontroller sleeps during LowPower.sleep() and wakes up on an interrupt or timer. The LED stays on during sleep because the pin output does not change. After waking, the program continues normally. This cycle repeats indefinitely, blinking the LED with sleep pauses in between. Key points are that sleep pauses CPU but not pins, wake-up resumes program, and sleep saves power between tasks.

Practice

1.

What is the main purpose of the LowPower library in Arduino?

easy

A. To connect Arduino to the internet

B. To increase the speed of the Arduino processor

C. To add more memory to the Arduino

D. To put the Arduino into sleep mode to save battery power

Solution

1. Step 1: Understand the LowPower library function

   The LowPower library is designed to reduce power consumption by putting the Arduino into sleep mode.

2. Step 2: Compare options with the library purpose

   Options B, C, and D do not relate to power saving or sleep mode, only To put the Arduino into sleep mode to save battery power matches the purpose.

3. Final Answer:

   To put the Arduino into sleep mode to save battery power -> Option D

4. Quick Check:

   LowPower library = Sleep mode for saving power [OK]

Hint: LowPower library = sleep mode to save battery [OK]

Common Mistakes:

• Thinking it speeds up the processor
• Confusing it with memory or internet functions
• Assuming it runs code faster

2.

Which of the following is the correct syntax to put Arduino to sleep for 8 seconds using the LowPower library?

LowPower._____();

easy

A. powerDown(SLEEP_8S, ADC_OFF, BOD_OFF)

B. sleep(8)

C. sleepMode(8)

D. sleepFor(8s)

Solution

1. Step 1: Recall LowPower sleep syntax

   The LowPower library uses powerDown() with parameters for sleep duration and power settings.

2. Step 2: Match correct parameters for 8 seconds sleep

   SLEEP_8S is the constant for 8 seconds, and ADC and BOD are turned off for power saving.

3. Final Answer:

   powerDown(SLEEP_8S, ADC_OFF, BOD_OFF) -> Option A

4. Quick Check:

   LowPower powerDown with SLEEP_8S = correct syntax [OK]

Hint: Use powerDown with SLEEP_8S for 8 seconds sleep [OK]

Common Mistakes:

• Using sleep(8) which is not a LowPower function
• Adding units like '8s' in function calls
• Using incorrect function names like sleepMode

3.

What will be the output on the serial monitor after running this code snippet?

#include <LowPower.h>

void setup() {
  Serial.begin(9600);
  Serial.println("Start");
  LowPower.powerDown(SLEEP_4S, ADC_OFF, BOD_OFF);
  Serial.println("Wake up");
}

void loop() {}

medium

A. Start\nWake up

B. Start

C. Wake up

D. No output

Solution

1. Step 1: Analyze setup() execution

   The code prints "Start", then sleeps for 4 seconds, then prints "Wake up".

2. Step 2: Understand sleep effect on Serial output

   Sleeping pauses the CPU but does not stop the program flow; after sleep, it continues to print "Wake up".

3. Final Answer:

   Start\nWake up -> Option A

4. Quick Check:

   Sleep pauses, then continues printing [OK]

Hint: Sleep pauses but code continues after wake [OK]

Common Mistakes:

• Thinking sleep stops all code permanently
• Expecting no output after sleep
• Confusing sleep with reset

4.

Identify the error in this code using the LowPower library:

#include <LowPower.h>

void setup() {
  LowPower.powerDown(SLEEP_1S, ADC_OFF);
}

void loop() {}

medium

A. powerDown should be called in loop(), not setup()

B. Incorrect library include syntax

C. Missing third parameter for powerDown function

D. SLEEP_1S is not a valid sleep duration

Solution

1. Step 1: Check powerDown function parameters

   The powerDown function requires three parameters: sleep duration, ADC state, and BOD state.

2. Step 2: Identify missing parameter

   The code only provides two parameters, missing the BOD_OFF or BOD_ON parameter.

3. Final Answer:

   Missing third parameter for powerDown function -> Option C

4. Quick Check:

   powerDown needs 3 params: duration, ADC, BOD [OK]

Hint: powerDown always needs 3 parameters [OK]

Common Mistakes:

• Omitting the BOD parameter
• Calling powerDown in setup is allowed
• Assuming SLEEP_1S is invalid

5.

You want your Arduino to sleep for 8 seconds repeatedly until a button press wakes it up. Which code snippet correctly uses the LowPower library to achieve this?

void loop() {
  // Sleep for 8 seconds
  LowPower.powerDown(_____, ADC_OFF, BOD_OFF);
  // Check button press here
}

hard

A. SLEEP_FOREVER

B. SLEEP_8S

C. SLEEP_16S

D. SLEEP_4S

Solution

1. Step 1: Identify sleep duration constant for 8 seconds

   The LowPower library defines SLEEP_8S for 8 seconds sleep intervals.

2. Step 2: Confirm repeated sleep in loop

   Calling powerDown with SLEEP_8S inside loop will sleep repeatedly for 8 seconds until interrupted.

3. Final Answer:

   SLEEP_8S -> Option B

4. Quick Check:

   Use SLEEP_8S for 8 seconds sleep [OK]

Hint: Use SLEEP_8S for 8 seconds sleep in loop [OK]

Common Mistakes:

• Using SLEEP_FOREVER which is not a valid duration
• Choosing invalid SLEEP_16S
• Choosing shorter sleep times like SLEEP_4S