Duty cycle control for motor/LED in Embedded C

Introduction

Duty cycle control helps change how strong or bright a motor or LED works by turning it on and off very fast.

To make an LED glow dimmer or brighter without changing the LED itself.

To control the speed of a small motor in a toy or gadget.

To save power by reducing how long a device is on.

To create effects like blinking lights or motor speed changes.

To adjust heating elements by controlling power delivery.

Syntax

Embedded C

void setDutyCycle(int dutyCyclePercent) {
    // dutyCyclePercent is from 0 to 100
    int compareValue = (MAX_TIMER_VALUE * dutyCyclePercent) / 100;
    TIMER_COMPARE_REGISTER = compareValue;
}

The duty cycle is usually given as a percentage from 0 (off) to 100 (fully on).

The timer compare register sets how long the output stays on in each cycle.

Examples

Sets the duty cycle to 0%, so the device is off.

Embedded C

setDutyCycle(0); // LED or motor off

Sets the duty cycle to 50%, so the device is on half the time.

Embedded C

setDutyCycle(50); // Half brightness or speed

Sets the duty cycle to 100%, so the device is fully on.

Embedded C

setDutyCycle(100); // Full brightness or speed

Sample Program

This program simulates setting the duty cycle for a motor or LED by calculating the timer compare register value and printing it.

Embedded C

#include <stdio.h>

#define MAX_TIMER_VALUE 1000

int TIMER_COMPARE_REGISTER = 0;

void setDutyCycle(int dutyCyclePercent) {
    if (dutyCyclePercent < 0) dutyCyclePercent = 0;
    if (dutyCyclePercent > 100) dutyCyclePercent = 100;
    int compareValue = (MAX_TIMER_VALUE * dutyCyclePercent) / 100;
    TIMER_COMPARE_REGISTER = compareValue;
    printf("Duty cycle set to %d%%, compare register = %d\n", dutyCyclePercent, TIMER_COMPARE_REGISTER);
}

int main() {
    setDutyCycle(0);    // Off
    setDutyCycle(25);   // 25% on
    setDutyCycle(50);   // 50% on
    setDutyCycle(75);   // 75% on
    setDutyCycle(100);  // Fully on
    return 0;
}

Important Notes

Duty cycle control uses a technique called PWM (Pulse Width Modulation).

Changing the duty cycle changes how long the device is powered in each cycle, affecting brightness or speed.

Always check your hardware limits to avoid damage when setting duty cycles.

Summary

Duty cycle controls how long a device is on in each cycle to adjust brightness or speed.

It is expressed as a percentage from 0 to 100.

Use a timer compare register to set the duty cycle in embedded systems.

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