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I'm comfortable with the physical side of circuitry etc but a total newbie at python so I've been messing around with a Raspberry Pi 4B and python 3 to help me understand how to control various aspects of components. Here I have a rgb LED and I want to cycle through full white light, red light, green light, and blue light before returning back to the begining of the loop. I finally have it working how I want it but I'm confused about HOW the loop is working: When I set a GPIO.output to 0, it's turning ON the corresponding pin. Logically, this makes no sense to me because I would expect that the value 1 would turn the pin on. I'm thinking I might be misunderstanding how the loop works so I'm hoping someone might be able to enlighten me? I've filled this code with a LOT of comments that explain my understanding of what each line is doing.

import time

redpin = 29                         # define redpin to 29
print ("redpin set to 29")
time.sleep(1.0)                     # 1 second delay
greenpin = 31                       # define greenpin to 31
print ("greenpin set to 31")
time.sleep(1.0)                     # 1 second delay
bluepin = 33                        # define bluepin to 33
print ("bluepin set to 33")
time.sleep(1.0)                     # 1 second delay

def setup():
    GPIO.setmode(GPIO.BOARD)        # use PHYSICAL GPIO numbering
    GPIO.setup(redpin,GPIO.OUT)     # set redpin to output mode
    GPIO.output(redpin,0)           # set redpin output to 0
    GPIO.setup(greenpin,GPIO.OUT)   # set greenpin to output mode
    GPIO.output(greenpin,0)         # set greenpin output to 0
    GPIO.setup(bluepin,GPIO.OUT)    # set bluepin to output mode
    GPIO.output(bluepin,0)          # set bluepin output to 0

def loop():
    while True:
        print("start of loop. no LED")
        GPIO.output(redpin,1)       # set redpin to 1 (off)
        GPIO.output(greenpin,1)     # set greenpin to 1 (off)
        GPIO.output(bluepin,1)      # set bluepin to 1 (off)
        time.sleep(1.5)             # 1.5 second delay
        print("White LED")
        GPIO.output(redpin,0)       # set redpin to 0 (on)
        GPIO.output(greenpin,0)     # set greenpin to 0 (on)
        GPIO.output(bluepin,0)      # set bluepin to 0 (on)
        time.sleep(4.0)             # 4 second delay
        print("no LED")
        GPIO.output(redpin,1)       # set redpin to 1 (off)
        GPIO.output(greenpin,1)     # set greenpin to 1 (off)
        GPIO.output(bluepin,1)      # set bluepin to 1 (off)
        time.sleep(1.5)             # 1.5 second delay
        print("red LED only")
        GPIO.output(redpin,0)       # set redpin to 0 (on)
        GPIO.output(greenpin,1)     # set green to 1 (off)
        GPIO.output(bluepin,1)      # set bluepin to 1 (off)
        time.sleep(4.0)             # 4 second delay
        print("no LED")
        GPIO.output(redpin,1)       # set redpin to 1 (off)
        GPIO.output(greenpin,1)     # set greenpin to 1 (off)
        GPIO.output(bluepin,1)      # set bluepin to 1 (off)
        time.sleep(1.5)             # 1.5 second delay
        print("green LED only")
        GPIO.output(redpin,1)       # set redpin to 1 (off)
        GPIO.output(greenpin,0)     # set greenpin to 0 (of)
        GPIO.output(bluepin,1)      # set bluepin to 1 (off)
        time.sleep(4.0)             # 4 second delay
        print("no LED")
        GPIO.output(redpin,1)       # set redpin to 1 (off)
        GPIO.output(greenpin,1)     # set greenpin to 1 (off)
        GPIO.output(bluepin,1)      # set bluepin to 1 (off)
        time.sleep(1.5)             # 1.5 second delay
        print("blue LED only")
        GPIO.output(redpin,1)       # set redpin to 1 (off)
        GPIO.output(greenpin,1)     # set greenpin to 1 (off)
        GPIO.output(bluepin,0)      # set bluepin to 0 (on)
        time.sleep(4.0)             # 4 second delay
        print("end of loop. returning to begining of loop.")
        time.sleep(1)               # 1 second delay

def destroy():
    GPIO.cleanup()                  # reset all GPIO to 0

if __name__ == '__main__':          # Program entrance
    print ("LED should be off")
    setup()
    try:
        loop()
    except KeyboardInterrupt:       # Press ctrl-c to end the program.
        destroy()

1

Presumably you have a common anode RGB LED. In these you connect 3V3 to the common anode and connect each leg via a resistor to a GPIO. Current flows when the GPIO is set to 0V and acts as a current sink.

Contrast with a common cathode RGB LED. In these you connect ground to the common cathode and connect each leg via a resistor to a GPIO. Current flows when the GPIO is set to 3V3 and acts as a current source.

| improve this answer | |
  • Yup, that's exactly it. I have a canakit and you can't really tell from looking at RGB LED but after reviewing your answer I went back and looked over a tutorial that accompanied the RGB LED. Sure enough I can see the common anode is hooked up to the 3.3Vdc supply. Thanks for the prompt and descriptive answer! – Bar May 9 at 22:35
  • @Bar Please accept the answer with a click on the tick on its left side. Only this will finish the question and it will not pop up again year for year. – Ingo May 10 at 10:57

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