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.