LED brightness control with PWM in Arduino - Step-by-Step Execution

Concept Flow - LED brightness control with PWM

Start

↓

Set PWM pin as OUTPUT

↓

Loop Start

↓

Increase brightness value

↓

Write PWM value to LED pin

↓

Delay for visible change

↓

If brightness > max?

Yes→Reset brightness to 0

↩Back to Loop Start

The program sets a pin for PWM output, then repeatedly increases brightness by changing PWM value, writes it to the LED, delays, and resets after max brightness.

Execution Sample

Arduino

int ledPin = 9;
int brightness = 0;
void setup() {
  pinMode(ledPin, OUTPUT);
}
void loop() {
  analogWrite(ledPin, brightness);
  brightness += 5;
  if (brightness > 255) brightness = 0;
  delay(30);
}

This code gradually increases LED brightness using PWM on pin 9, then resets to 0 to repeat the cycle.

Execution Table

Step	brightness	Condition brightness > 255?	Action	PWM Output to LED
1	0	No	Write PWM 0, brightness += 5	0
2	5	No	Write PWM 5, brightness += 5	5
3	10	No	Write PWM 10, brightness += 5	10
4	15	No	Write PWM 15, brightness += 5	15
5	20	No	Write PWM 20, brightness += 5	20
...	...	...	...	...
52	255	No	Write PWM 255, brightness += 5	255
53	260	Yes	Reset brightness to 0	260 (not written)
54	0	No	Write PWM 0, brightness += 5	0

💡 Loop runs indefinitely; brightness resets to 0 after exceeding 255.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	...	After 52	After 53	After 54
brightness	0	5	10	15	20	25	...	255	0	5

Key Moments - 3 Insights

Why does brightness reset to 0 after 255?

Because PWM values must be between 0 and 255. When brightness exceeds 255 (see step 53 in execution_table), it resets to 0 to start the cycle again.

Why do we add 5 to brightness each loop?

Adding 5 increases brightness gradually for a smooth fade effect. This is shown in the action column of execution_table where brightness increments by 5 each step.

What happens if delay(30) is removed?

Without delay, brightness changes too fast to see the fade effect clearly. The LED would appear to jump brightness instantly instead of smoothly.

Visual Quiz - 3 Questions

Look at the execution_table, what is the brightness value at step 4?

A 10

B 15

C 20

D 5

💡 Hint

Check the brightness column at step 4 in the execution_table.

At which step does brightness reset to 0?

A Step 52

B Step 54

C Step 53

D Step 50

💡 Hint

Look for the row where condition brightness > 255 is Yes and action resets brightness.

If we change brightness increment from 5 to 10, how does it affect the execution_table?

A Brightness increases faster, fewer steps before reset

B Brightness increases slower, more steps before reset

C Brightness stays the same, no change

D Brightness resets at 5 instead of 255

💡 Hint

Increasing increment means brightness reaches 255 quicker, so fewer steps before reset.

Concept Snapshot

Arduino PWM controls LED brightness by sending values 0-255.
Use analogWrite(pin, value) to set brightness.
Increase value gradually in loop for fade effect.
Reset value after 255 to repeat.
Add delay for visible smooth change.

Full Transcript

This example shows how to control LED brightness on Arduino using PWM. The program sets a pin as output and uses analogWrite to send brightness values from 0 to 255. Each loop increases brightness by 5, writes it to the LED, then delays 30 milliseconds. When brightness goes above 255, it resets to 0 to start fading again. The execution table traces each step, showing brightness values, conditions, and actions. Key moments explain why brightness resets and why delay is important. The quiz tests understanding of brightness values at steps and effects of changing increment.

Practice

1. What does analogWrite(pin, 128) do to an LED connected to the specified pin?

easy

A. Sets the LED brightness to about half of its maximum brightness.

B. Turns the LED fully off.

C. Turns the LED fully on at maximum brightness.

D. Causes the LED to blink on and off repeatedly.

Solution

1. Step 1: Understand PWM value range

   The analogWrite function accepts values from 0 (off) to 255 (full brightness).

2. Step 2: Interpret the value 128

   128 is about half of 255, so the LED brightness will be about half of maximum.

3. Final Answer:

   Sets the LED brightness to about half of its maximum brightness. -> Option A

4. Quick Check:

   PWM value 128 = half brightness [OK]

Hint: Remember 0=off, 255=full brightness, 128=half brightness [OK]

Common Mistakes:

• Thinking 128 turns LED off
• Confusing analogWrite with digitalWrite
• Assuming 128 means blinking

2. Which of the following is the correct syntax to set PWM brightness on pin 9 to maximum in Arduino?

easy

A. analogWrite(255, 9);

B. digitalWrite(9, 255);

C. analogWrite(9, 255);

D. analogRead(9, 255);

Solution

1. Step 1: Recall analogWrite syntax

   The correct syntax is analogWrite(pin, value) where pin is the pin number and value is 0-255.

2. Step 2: Check each option

   analogWrite(9, 255); uses analogWrite(9, 255); which is correct. Others have wrong function names or argument order.

3. Final Answer:

   analogWrite(9, 255); -> Option C

4. Quick Check:

   Correct function and argument order = analogWrite(9, 255); [OK]

Hint: analogWrite(pin, value) sets PWM; pin first, value second [OK]

Common Mistakes:

• Swapping pin and value arguments
• Using digitalWrite instead of analogWrite
• Using analogRead instead of analogWrite

3. What will be the effect of this Arduino code snippet on an LED connected to pin 6?

for (int brightness = 0; brightness <= 255; brightness += 51) {
  analogWrite(6, brightness);
  delay(100);
}

medium

A. The LED will stay at full brightness without change.

B. The LED will blink on and off 5 times quickly.

C. The code will cause a compile error due to wrong syntax.

D. The LED brightness will increase in 5 steps from off to full brightness.

Solution

1. Step 1: Analyze the for loop increments

   The loop variable brightness starts at 0 and increases by 51 until it reaches 255, so values are 0, 51, 102, 153, 204, 255 (6 values, 5 steps).

2. Step 2: Understand analogWrite effect

   Each loop sets LED brightness to the current value, increasing brightness in steps with 100ms delay.

3. Final Answer:

   The LED brightness will increase in 5 steps from off to full brightness. -> Option D

4. Quick Check:

   Brightness steps with delay = gradual brightness increase [OK]

Hint: Loop increments PWM value to fade LED brightness up [OK]

Common Mistakes:

• Counting 6 steps instead of 5 increments
• Thinking LED blinks on/off instead of fading
• Assuming syntax error due to loop

4. Identify the error in this Arduino code that tries to fade an LED on pin 10:

for (int i = 0; i < 256; i++) {
  analogWrite(10, i);
  delay(50);
}
analogWrite(10, 256);

medium

A. The for loop should use <= 256 instead of < 256.

B. The value 256 is invalid for analogWrite; max is 255.

C. Pin 10 cannot be used with analogWrite on Arduino.

D. delay() cannot be used inside a for loop.

Solution

1. Step 1: Check analogWrite value limits

   analogWrite accepts values from 0 to 255. Using 256 is out of range and invalid.

2. Step 2: Verify other code parts

   The for loop correctly uses i from 0 to 255. Pin 10 supports PWM on most Arduino boards. delay() is allowed inside loops.

3. Final Answer:

   The value 256 is invalid for analogWrite; max is 255. -> Option B

4. Quick Check:

   analogWrite max value = 255, 256 causes error [OK]

Hint: PWM values max at 255; never use 256 [OK]

Common Mistakes:

• Using values above 255 for analogWrite
• Thinking delay() is not allowed in loops
• Assuming pin 10 can't do PWM

5. You want to create a smooth breathing LED effect using PWM on pin 3. Which code snippet correctly achieves this effect?

hard

A. for (int b = 0; b <= 255; b++) { analogWrite(3, b); delay(10); } for (int b = 255; b >= 0; b--) { analogWrite(3, b); delay(10); }

B. analogWrite(3, 255); delay(1000); analogWrite(3, 0); delay(1000);

C. for (int b = 0; b < 256; b += 50) { analogWrite(3, b); delay(100); }

D. digitalWrite(3, HIGH); delay(500); digitalWrite(3, LOW); delay(500);

Solution

1. Step 1: Understand breathing LED effect

   A breathing effect smoothly increases brightness from 0 to max, then back down to 0 repeatedly.

2. Step 2: Analyze each option

   for (int b = 0; b <= 255; b++) { analogWrite(3, b); delay(10); } for (int b = 255; b >= 0; b--) { analogWrite(3, b); delay(10); } uses two loops: one increasing PWM from 0 to 255, then decreasing back to 0 with small delays for smoothness. analogWrite(3, 255); delay(1000); analogWrite(3, 0); delay(1000); just turns LED fully on and off abruptly. for (int b = 0; b < 256; b += 50) { analogWrite(3, b); delay(100); } increases brightness in large steps, not smooth. digitalWrite(3, HIGH); delay(500); digitalWrite(3, LOW); delay(500); uses digitalWrite, which only turns LED fully on or off.

3. Final Answer:

   for (int b = 0; b <= 255; b++) { analogWrite(3, b); delay(10); } for (int b = 255; b >= 0; b--) { analogWrite(3, b); delay(10); } -> Option A

4. Quick Check:

   Smooth increase and decrease PWM = breathing LED [OK]

Hint: Use increasing then decreasing PWM values for breathing effect [OK]

Common Mistakes:

• Using digitalWrite instead of analogWrite for brightness
• Skipping the decreasing brightness loop
• Using large PWM steps causing jerky effect