LED brightness control with PWM in Arduino - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the LED brightness after running this code?

Consider this Arduino code that sets LED brightness using PWM:

int ledPin = 9;
void setup() {
  pinMode(ledPin, OUTPUT);
}
void loop() {
  analogWrite(ledPin, 128);
  delay(1000);
}

What brightness level will the LED have?

Arduino

int ledPin = 9;
void setup() {
  pinMode(ledPin, OUTPUT);
}
void loop() {
  analogWrite(ledPin, 128);
  delay(1000);
}

A LED brightness is about 50% of maximum

B LED brightness is at 100% (fully on)

C LED brightness is off (0%)

D LED brightness is about 25% of maximum

💡 Hint

PWM values range from 0 (off) to 255 (fully on). 128 is about half of 255.

The analogWrite function sets the PWM duty cycle. A value of 128 is roughly half of 255, so the LED will be about half as bright as fully on.

🧠 Conceptual

intermediate

Why use PWM to control LED brightness?

Which of the following best explains why PWM is used to control LED brightness instead of just changing voltage?

A PWM allows controlling brightness by switching LED on and off rapidly, which is efficient and prevents damage.

B PWM increases the LED's power consumption.

C PWM reduces the LED's color temperature.

D Changing voltage directly is easier and more efficient than PWM.

💡 Hint

Think about how LEDs respond to voltage and how PWM works.

LEDs are either on or off at a given voltage. PWM controls brightness by turning the LED on and off very fast, changing the time it is on versus off, which is efficient and safe.

🔧 Debug

advanced

Why does this LED brightness code not work as expected?

Look at this Arduino code snippet:

int ledPin = 9;
void setup() {
  pinMode(ledPin, OUTPUT);
  analogWrite(ledPin, 300);
}
void loop() {
}

The LED does not light up as expected. What is the problem?

A The <code>analogWrite</code> function must be called repeatedly in <code>loop()</code>.

B The LED pin 9 cannot be used for PWM.

C The <code>pinMode</code> function should be called in <code>loop()</code>.

D The PWM value 300 is out of range; it must be between 0 and 255.

💡 Hint

Check the valid range for PWM values in Arduino.

Arduino PWM values must be between 0 and 255. Using 300 is invalid and causes unexpected behavior.

📝 Syntax

advanced

Identify the syntax error in this PWM code

Find the syntax error in this Arduino code:

int ledPin = 9
void setup() {
  pinMode(ledPin, OUTPUT);
}
void loop() {
  analogWrite(ledPin, 128);
  delay(1000);
}

A Missing parentheses in <code>pinMode</code> call

B Missing semicolon after <code>int ledPin = 9</code>

C Incorrect function name <code>analogWrite</code>

D Missing curly braces in <code>loop</code> function

💡 Hint

Check the end of the first line carefully.

The first line is missing a semicolon at the end, which is required in Arduino C++ syntax.

🚀 Application

expert

How to smoothly increase LED brightness using PWM?

You want to write Arduino code to smoothly increase LED brightness from off to fully on over 5 seconds using PWM on pin 9. Which code snippet achieves this?

A analogWrite(9, 255); delay(5000);

B for (int i = 255; i >= 0; i--) { analogWrite(9, i); delay(20); }

C for (int i = 0; i <= 255; i++) { analogWrite(9, i); delay(20); }

D for (int i = 0; i <= 255; i += 5) { analogWrite(9, i); delay(100); }

💡 Hint

Think about how to increase brightness gradually with small steps and short delays.

Option C increases PWM value from 0 to 255 in steps of 1 with 20ms delay, totaling about 5 seconds for smooth brightness increase.

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