Duty cycle control for motor/LED in Embedded C - Mini Project: Build & Apply

Duty cycle control for motor/LED

📖 Scenario: You are working on a simple embedded system that controls the brightness of an LED or the speed of a motor using PWM (Pulse Width Modulation). The duty cycle determines how long the signal stays ON in one cycle, affecting brightness or speed.In this project, you will write code to set up the duty cycle and then calculate the ON time based on a fixed period.

🎯 Goal: Build a program that defines a duty cycle percentage, calculates the ON time for a PWM signal with a fixed period, and prints the ON time value.

📋 What You'll Learn

Create a variable to hold the PWM period in microseconds

Create a variable to hold the duty cycle percentage

Calculate the ON time based on the duty cycle and period

Print the ON time value

💡 Why This Matters

🌍 Real World

PWM signals are used in many devices to control motor speed, LED brightness, and other electronics by adjusting the duty cycle.

💼 Career

Understanding duty cycle control is important for embedded systems engineers and anyone working with hardware control and microcontrollers.

1

Set the PWM period

Create an integer variable called pwm_period and set it to 20000 to represent the PWM period in microseconds.

Embedded C

// Create an integer variable called pwm_period and set it to 20000
// Your code here

Hint

The PWM period is the total time of one cycle. Use an integer variable named pwm_period and assign it the value 20000.

2

Set the duty cycle percentage

Create an integer variable called duty_cycle and set it to 30 to represent the duty cycle percentage (30%).

Embedded C

int pwm_period = 20000;
// Create an integer variable called duty_cycle and set it to 30
// Your code here

Hint

The duty cycle is the percentage of time the signal is ON. Use an integer variable named duty_cycle and assign it the value 30.

3

Calculate the ON time

Create an integer variable called on_time and calculate its value as (pwm_period * duty_cycle) / 100 to find the ON time in microseconds.

Embedded C

int pwm_period = 20000;
int duty_cycle = 30;
// Calculate the ON time and store it in an integer variable called on_time
// Your code here

Hint

Multiply pwm_period by duty_cycle and divide by 100 to get the ON time.

4

Print the ON time

Use printf to print the text "ON time: " followed by the value of on_time and then " microseconds\n".

Embedded C

int pwm_period = 20000;
int duty_cycle = 30;
int on_time = (pwm_period * duty_cycle) / 100;
// Print the ON time value
// Your code here

Hint

Use printf("ON time: %d microseconds\n", on_time); to display the ON time.

Practice

1. What does the duty cycle control in a motor or LED?

easy

A. The percentage of time the device is ON during each cycle

B. The voltage supplied to the device

C. The frequency of the power supply

D. The temperature of the device

Solution

1. Step 1: Understand duty cycle meaning

   The duty cycle is the fraction of one period in which a signal or system is active.

2. Step 2: Relate to motor/LED control

   For motors or LEDs, it controls how long the device is ON in each cycle, affecting speed or brightness.

3. Final Answer:

   The percentage of time the device is ON during each cycle -> Option A

4. Quick Check:

   Duty cycle = ON time percentage [OK]

Hint: Duty cycle means ON time percentage per cycle [OK]

Common Mistakes:

• Confusing duty cycle with voltage
• Thinking it controls frequency
• Assuming it controls temperature

2. Which of the following is the correct way to set a 50% duty cycle using a timer compare register in embedded C?

easy

A. OCR0 = 0; // Full OFF

B. OCR0 = 255; // Full ON

C. OCR0 = 128; // Assuming 8-bit timer with max 255

D. OCR0 = 64; // 25% duty cycle

Solution

1. Step 1: Understand timer resolution

   An 8-bit timer counts from 0 to 255, so max value is 255.

2. Step 2: Calculate 50% duty cycle value

   50% of 255 is approximately 128, so setting OCR0 to 128 sets 50% duty cycle.

3. Final Answer:

   OCR0 = 128; // Assuming 8-bit timer with max 255 -> Option C

4. Quick Check:

   50% of 255 = 128 [OK]

Hint: Half of max timer value sets 50% duty cycle [OK]

Common Mistakes:

• Using max value for 50% duty cycle
• Using zero for 50% duty cycle
• Confusing 25% with 50%

3. Given the code snippet below, what will be the duty cycle percentage set for the LED?

unsigned char duty = 64;
unsigned char max = 255;
float dutyCycle = (duty / (float)max) * 100;

medium

A. 25%

B. 50%

C. 75%

D. 64%

Solution

1. Step 1: Calculate duty/max ratio

   Divide duty (64) by max (255): 64 / 255 ≈ 0.25098.

2. Step 2: Convert ratio to percentage

   Multiply by 100: 0.25098 * 100 ≈ 25.1%.

3. Final Answer:

   25% -> Option A

4. Quick Check:

   (64/255)*100 ≈ 25% [OK]

Hint: Divide duty by max, multiply by 100 for percentage [OK]

Common Mistakes:

• Using integer division without casting
• Confusing duty value with percentage
• Rounding incorrectly

4. Identify the error in the following code that attempts to set a 75% duty cycle on an 8-bit timer:

unsigned char duty = 75;
unsigned char max = 255;
OCR0 = duty / max * 255;

medium

A. OCR0 register cannot be assigned directly

B. Integer division causes duty/max to be zero

C. max value should be 100, not 255

D. Duty cycle calculation is correct

Solution

1. Step 1: Analyze division operation

   Both duty and max are unsigned char, so duty/max does integer division: 75/255 = 0 (integer).

2. Step 2: Effect on OCR0 assignment

   Multiplying 0 by 255 results in 0, so OCR0 is set to 0, not 75% duty cycle.

3. Final Answer:

   Integer division causes duty/max to be zero -> Option B

4. Quick Check:

   Integer division truncates to zero [OK]

Hint: Cast to float before division to avoid zero result [OK]

Common Mistakes:

• Ignoring integer division rules
• Assuming OCR0 cannot be assigned
• Using wrong max value

5. You want to smoothly increase the brightness of an LED from 0% to 100% duty cycle using PWM on an 8-bit timer. Which approach is best to implement this in embedded C?

hard

A. Use a fixed OCR0 value of 128 for half brightness

B. Set OCR0 to 255 once and leave it

C. Toggle the LED pin ON and OFF manually without PWM

D. Use a loop to increment OCR0 from 0 to 255 with a delay between steps

Solution

1. Step 1: Understand smooth brightness control

   Smooth brightness change requires gradually changing the duty cycle from 0 to max.

2. Step 2: Implement gradual PWM change

   Increment OCR0 from 0 to 255 in a loop with delays to create smooth brightness ramp.

3. Final Answer:

   Use a loop to increment OCR0 from 0 to 255 with a delay between steps -> Option D

4. Quick Check:

   Gradual OCR0 change = smooth brightness [OK]

Hint: Increment OCR0 gradually with delay for smooth fade [OK]

Common Mistakes:

• Setting OCR0 once for full brightness only
• Using fixed mid value for all brightness
• Not using PWM for brightness control