Lesson 3 Breathing LED

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In this lesson, we will try something interesting – gradually increase or decrease the luminance of an LED with PWM, just like breathing. So we give it a magical name – Breathing LED.

Newly Added Components


Pulse Width Modulation, or PWM, is a technique for getting analog results with digital means. Digital control is used to create a square wave, a signal switched between on and off. This on-off pattern can simulate voltages in between full on (5 Volts) and off (0 Volts) by changing the portion of the time the signal spends on versus the time that the signal spends off. The duration of “on time” is called pulse width. To get varying analog values, you can change or modulate this width. If you repeat this on-off pattern fast enough with some device, an LED for example, the result would be like this: the signal is a steady voltage between 0 and 5v controlling the brightness of the LED.

Duty Cycle

A duty cycle is the percentage of one period in which a signal is active. A period is the time it takes for a signal to complete an on-and-off cycle. As a formula, a duty cycle may be expressed as:


Where is the duty cycle, is the time the signal is active, and is the total period of the signal. Thus, a 60% duty cycle means the signal is on 60% of the time but off 40% of the time. The “on time” for a 60% duty cycle could be a fraction of a second, a day, or even a week, depending on the length of the period.

Schematic Diagram

Build the Circuit

For C Language Users:


  1. Go to the folder of the code.

cd /home/pi/electronic-kit/for-raspberry-pi/c/Lesson_3_BreathingLed

  1. Compile the code.

gcc 3_BreathingLed.c -lwiringPi

  1. Run the executable file.

sudo ./a.out

As the code runs, you can see that the brightness of the LED becomes stronger or weaker.


1.#include <stdio.h>  
2.#include <wiringPi.h>  
3.#include <softPwm.h>
5.#define LedPin 1   
7.int main (void)  
9.// When initialize wiring failed, print message to screen  
10.    if(wiringPiSetup() == -1){  
11.        printf("setup wiringPi failed !");  
12.        return 1;   
13.    }  
15.    softPwmCreate(LedPin,  0, 100);  
17.    int i;  
19.    while(1) // loop forever  
20.    {  
21.        for(i=0;i=0;i--){  
27.            softPwmWrite(LedPin, i);  
28.            delay(10);  
29.        }   
30.    }  
31.    return 0 ;  

Code Explanation

  1. #include

WiringPi includes a software-driven PWM library of outputting a PWM signal on any of the Raspberry Pi’s GPIO pins. To maintain a low CPU usage, the minimum pulse width is 100μS. That combined with the default suggested range of 100 gives a PWM frequency of 100Hz. Within these limitations, control of a light/LED or a motor is very achievable.

  1. softPwmCreate(LedPin, 0, 100);

The function is to use software library to create a PWM pin, set its period between 0x100us-100x100us.

The prototype of the function softPwmCreate(LedPinRed, 0, 100) is as follows:

int softPwmCreate(int pin,int initialValue,int pwmRange);

pin: Any GPIO pin of Raspberry Pi can be set as a PWM pin.

initialValue: The initial pulse width is that initialValue times100us.

pwmRange: The period of PWM is that pwmRange times100us.

  1. softPwmWrite(LedPin, i);

The function is used to write the PWM value i to the LedPin.

The prototype of the function softPwmWrite(LedPinBlue, b_val) is as follows:

void softPwmWrite (int pin, int value) ;

pin: Any GPIO pin of Raspberry Pi can be set as a PWM pin.

Value: The pulse width of PWM is value times 100us. Note that value can only be less than pwmRange defined previously, if it is larger than pwmRange, the value will be given a fixed value, pwmRange.

  1. delay(10);

Wait for 10ms, interval time between the changes indicates the speed of breathing.

For Python Language Users


  1. Go to the folder of the code.

cd /home/pi/electronic-kit/for-raspberry-pi/python

  1. Run the code.

sudo python3 3_BreathingLed.py

As the code runs, you can see that the brightness of the LED becomes stronger or weaker.


1.import RPi.GPIO as GPIO  
2.import time  
4.LedPin = 18  
6.def setup():  
7.    global pLed  
8.    GPIO.setmode(GPIO.BCM)  
9.    GPIO.setup(LedPin, GPIO.OUT, initial=GPIO.LOW)  
10.    pLed = GPIO.PWM(LedPin, 1000)  
11.    pLed.start(0)  
13.def main():  
14.    # Set increase/decrease step  
15.    step =2   
16.    delay = 0.05  
17.    while True:  
18.        # Increase duty cycle from 0 to 100  
19.        for dc in range(0, 101, step):  
20.            pLed.ChangeDutyCycle(dc)  
21.            print (' ++ Duty cycle: %s'%dc)  
22.            time.sleep(delay)  
23.        time.sleep(1)  
25.        # decrease duty cycle from 100 to 0  
26.        for dc in range(100, -1, -step):  
27.            # Change duty cycle to dc  
28.            pLed.ChangeDutyCycle(dc)  
29.            print ('  -- Duty cycle: %s'%dc)  
30.            time.sleep(delay)  
31.        time.sleep(1)  
33.def destroy():  
34.    # Stop pLed  
35.    pLed.stop()  
36.    # Turn off LED  
37.    GPIO.output(LedPin, GPIO.LOW)  
38.    # Release resource  
39.    GPIO.cleanup()  
41.# If run this script directly, do:  
42.if __name__ == '__main__':  
43.    setup()  
44.    try:  
45.        main()  
46.    # When 'Ctrl+C' is pressed, the child program   
47.    # destroy() will be  executed.  
48.    except KeyboardInterrupt:  
49.        destroy()  

Code Explanation

  1. pLed = GPIO.PWM(LedPin, 1000) To create a PWM instance. Set pLed as pwm output and frequence to 1KHz.
  2. pLed.start(0)

Set pLed begin with value 0.

  1. for dc in range(0, 101, step):
  2. # Change duty cycle to dc
  3. pLed.ChangeDutyCycle(dc)
  4. print (‘ ++ Duty cycle: %s’%dc)
  5. time.sleep(delay)

Increase the duty cycle by 2 at a time, from 0 to 101, and you’ll see the LED getting brighter and brighter.

  1. for dc in range(100, -1, -step):
  2. pLed.ChangeDutyCycle(dc)
  3. print (‘ — Duty cycle: %s’%dc)
  4. time.sleep(delay)

Similarly, when the duty cycle is reduced by 2 from 100 to -1, the LED brightness will be dimmer and dimmer.