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I am using hw-479 that looks like the picture below. I cannot find much of information for it but the Alibaba page says

The RGB LED module is made of a plug-in full-color LED. The PWM voltage input of the three pins of R G and B can adjust the intensity of three primary colors (red/blue/green) to achieve full-color color mixing. The Arduino's control of the module enables cool lighting effects.

Of course, since I am using a Pi, I cannot use that Arduino library. I wrote the following code, but it does not work. Turning individual colours by setting GPIO.output(pin, HIGH) was successful, so I do not think the pin connections are wrong.

I wonder what is wrong with the code. First, I do not understand what the frequency should be. The example was using 50, so I used 50. Secondly, it seems that the dc, the parameter of start() ranges from 0, to 100, and 0 means the maximum power and 100 means the minimum power. Is that correct?

import RPi.GPIO as GPIO
import time

redPin = 11
greenPin = 13
bluePin = 15

def setValue(pin, value):
    GPIO.setmode(GPIO.BOARD)
    GPIO.setup(pin, GPIO.OUT)
    p = GPIO.PWM(pin, 50)
    
    if 0>value or value>255:
        raise Exception("0 to 255")
    
    #dc = 0 max, dc = 100 min
    dc = 100 - (float(value)/256.*100);
    p.start(dc)
    
def setRGB(r,g,b):
    setValue(redPin, r)
    setValue(greenPin, g)
    setValue(bluePin, b)
    
if __name__ == "__main__":
    print("Salmon")
    setRGB(0xFA, 0x80, 0x72)
    raw_input()
    
    print("Dark slate gray")
    setRGB(0x2F, 0x4F, 0x4F)
    raw_input()
    
    GPIO.cleanup()

enter image description here

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1 Answer 1

Reset to default
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RPi.GPIO dutycycles are in the range 0 to 100 and represent a percentage of total power. So a dutycycle of 0 is off, 100 is fully on, and 50 is half power.

There is quite a lot of changes I would make to the code. Basically I would have an initialisation section where all the stuff which should only be done once would be placed.

    GPIO.setmode(GPIO.BOARD)

    GPIO.setup(redPin, GPIO.OUT)
    GPIO.setup(greenPin, GPIO.OUT)
    GPIO.setup(bluePin, GPIO.OUT)
    
    pR = GPIO.PWM(redPin, 50)
    pG = GPIO.PWM(greenPin, 50)
    pB = GPIO.PWM(bluePin, 50)
    
    pR.start(0)
    pG.start(0)
    pB.start(0)

I would use pR.ChangeDutyCycle(), pG.ChangeDutyCycle(), and pB.ChangeDutyCycle() to update the dutcycles.

See https://sourceforge.net/p/raspberry-gpio-python/wiki/PWM/

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  • It seems that if I create p like p=GPIO.PWM() inside of a function, the LED gets turned off once the control flow goes out of the function. I put the p in a global dictionary (if it exists, calls stop()) and then it did not turn off. Is it the best way to keep the returned object of the PWM() function in a global variable? Also, why 50 for the frequency? I had already seen the linked documentation, but it really did not give much of the details...
    – Damn Vegetables
    Jul 17, 2020 at 17:03
  • When you create the object (p) within a function it is deleted when it goes out of scope (i.e. as soon as the function returns). 50 Hz is a reasonable frequency but something like 500 would be better. The problem is that requires more CPU as RPi.GPIO uses software timing for the PWM.
    – joan
    Jul 17, 2020 at 17:12

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