Pi GPIO PWD LED

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How to change the brightness of an LED using the Pi's GPIO pins?

Based on: https://www.admfactory.com/breathing-light-led-on-raspberry-pi-using-python/

setup and testing

Start by following the basic setup and testing of the Pi's GPIO in RaspberryPi_GPIO


PWD

The way to change the brightness of an LED in digital electronics (only binary values are used 0v-5V or 0V-3.3V and no values in between), consists in creating a square waves that changes rapidly between 0V and 3.3V. By changing the portion of the time when the signal is on (3.3V) versus the time that the signal is off(0V) we can change the intensity of the LED. This technique is called Pulse Width Modulation (PWM), as we are modulating the "on" width of the wave.

Pwm.png

Circuit

Connect the LED to pin 12, as in the following schematic. Taking care with the LED polarity

BreathingLed01.png

Code

increasing / decreasing LED intensity

#!/usr/bin/env python3                                                                                
import RPi.GPIO as GPIO
import time

LedPin = 12

GPIO.setmode(GPIO.BOARD)       # Numbers pins by physical location    ## or GPIO.BCM                             
GPIO.setup(LedPin, GPIO.OUT)   # Set pin mode as output                                               
GPIO.output(LedPin, GPIO.LOW)  # Set pin initial value to low(0V)                                     

p = GPIO.PWM(LedPin, 1000)     # set Frequece to 1KHz                                                 
p.start(0)                     # Start PWM output, Duty Cycle = 0                                     

pwd_change_time = 0.1 # time it waits at every step of DutyCycle                                      

try:
    # loop indefinitly through 2 sub loops (increasing and decreasing duty cyle-brightness):          
    while True:
        for dc in range(0, 101, 5):   # Increase duty cycle: from  0 to 100, in steps of 5            
            p.ChangeDutyCycle(dc)     # Change duty cycle                                             
            time.sleep( pwd_change_time ) # wait to get to next value                                 
        time.sleep(1)
        for dc in range(100, -1, -5): # Decrease duty cycle: from  100 to 0, in steps of 5            
            p.ChangeDutyCycle(dc)
            time.sleep( pwd_change_time )
        time.sleep(1)
except KeyboardInterrupt: #Ctl-c will interrup                                                        
    p.stop()
    GPIO.output(LedPin, GPIO.LOW) # turn off led                                                      
    GPIO.cleanup() # clean up resources

random LED intensity

#!/usr/bin/env python3                                                                                
import RPi.GPIO as GPIO
import time
from random import randint
LedPin = 12

GPIO.setmode(GPIO.BOARD)       # Numbers pins by physical location     ## or GPIO.BCM                                 
GPIO.setup(LedPin, GPIO.OUT)   # Set pin mode as output                                               
GPIO.output(LedPin, GPIO.LOW)  # Set pin initial value to low(0V)                                     

p = GPIO.PWM(LedPin, 1000)     # set Frequece to 1KHz                                                 
p.start(0)                     # Start PWM output, Duty Cycle = 0                                     

pwd_change_time = 0.1 # time it waits at every step of DutyCycle                                      
dc_end=0

try:
    # loop indefinitly:                                                                               
    while True:
        dc_start = dc_end # start at value led was left of                                            
        dc_end = randint(0,100)   # select random number between 0 and 100                            

        if dc_start > dc_end: # if start value > end value: range step has to be negative             
            step = - 5
        else:
            step = 5

        for dc in range(dc_start, dc_end, step):
            p.ChangeDutyCycle(dc)     # Change duty cycle                                             
            time.sleep(pwd_change_time )

        time.sleep(1)

except KeyboardInterrupt: #Ctl-c will interrup                                                        
    p.stop()
    GPIO.output(LedPin, GPIO.LOW) # turn off led                                                      
    GPIO.cleanup() # clean up resources