GPIO.output() works, GPIO.PWM() does not

Question

I'm working on a project with a RGB LED which has to indicate what the status of the internet connection is. It is running as a service. I'm focused on two files, one to check if there is an internet connection and one for controlling the LED.

The following code is where we check if there is an internet connection and trying to call the LED function:

import logging

from status_server import status_led

def testInternetConnection():
    logging.info("Testing the internet connection...")

    while True:
        try:
            r = requests.get(https://google.com)

            # HTTP 200 OK
            if r.status_code == 200:
                logging.info("Internet connection is OK!")
                status_led.turn_green()
                break
        except KeyboardInterrupt:
            sys.exit()
        except requests.exceptions.RequestException:
            logging.warning("Re-trying the internet connection....")
            status_led.turn_red()
            time.sleep(10)

It is working fine with the following code in the LED file:

import RPi.GPIO as GPIO

class _RGBLed:
    def __init__(self, red_pin, green_pin, blue_pin):
        self.RED_PIN = red_pin
        self.GREEN_PIN = green_pin

    self.init_pins()

    def init_pins(self):
        GPIO.setmode(GPIO.BCM)
        GPIO.setup(self.RED_PIN, GPIO.OUT)
        GPIO.setup(self.GREEN_PIN, GPIO.OUT)

    def turn_red(self)
        GPIO.output(self.RED_PIN, GPIO.HIGH)

    def turn_green(self):
        GPIO.output(self.GREEN_PIN, GPIO.HIGH)


_rgb_led = _RGBLed(red_pin=26, green_pin=19)


def turn_red():
    _rgb_led.turn_red()


def turn_green():
    _rgb_led.turn_green()

The LED is quite bright so I wanted to add PWM to control the LED and lower the duty cycle. I've tested a bit with PWM in the Python live environment and that wasn't that hard. I've changed the turn_green and turn_red functions with the following:

def turn_green():
    turn_green_on = GPIO.PWM(GREEN_PIN, 100)
    turn_green_on.start(20)

def turn_red():
    turn_red_led_on = GPIO.PWM(RED_PIN, 100)
    turn_red_led_on.start(20)

I'm calling those functions from another When I do this it says name GREEN_PIN is not defined. When I change GREEN_PIN to 19 it doesn't give me warnings anymore, but the LED does nothing too. I've checked if the function is called by adding a log statement so that's not the problem either.

Can someone help me with this? Thanks in advance :)

Answer 1

Per PWM's Wiki: (https://sourceforge.net/p/raspberry-gpio-python/wiki/PWM/)

Note that PWM will also stop if the instance variable 'p' goes out of scope.

Your variables are going out of scope (the scope of the method) as soon as you call it, to make them "permanent" basically in your class make both turn_green_on and turn_red_led_on instance variables, most likely defined in your class constructor:

def __init__(self):
    self._turn_green_on = None
    self._turn_red_led_on = None
    ...

And in your class' method:

def turn_green():
   self._turn_green_on = GPIO.PWM(GREEN_PIN, 0.5)
   self._turn_green_on.start(20)

Answer 2

If I understand it correctly, your PWM-frequency is 100Hz and your duty cycle is 20%. This would mean, that your LED is on for 1s/100*0.2 = 2ms and off for about 1s/100*0.8 = 8ms (and so on). Are you sure you can see that? Maybe you should start with

def turn_green():
   turn_green_on = GPIO.PWM(GREEN_PIN, 0.5)
   turn_green_on.start(20)

to be sure to be able to visually detect a working setup.