What does setting output high or low mean in RPi.GPIO

Question

If I setup a pin's output to HIGH or LOW, would current flow through it or not?

In this example, when I set output to HIGH then the LED turns off. I expected the other way around.

The python code I have written is

import RPi.GPIO as GPIO
import time

try:
    outputPins = [11,12,15,16,18,22,3,5,24]
    GPIO.setmode(GPIO.BOARD)

    for pin in outputPins:
        print('setting output mode for pin',pin)
        GPIO.setup(pin,GPIO.OUT)

    while True:
        for pin in outputPins:
            #print('turning ON pin ',pin)
            GPIO.output(pin,GPIO.LOW) <-- I expected that HIGH will turn the LED ON
            time.sleep(0.1)
            #print('turning OFF pin ',pin)
            GPIO.output(pin,GPIO.HIGH) <-- I expected low to turn LED off
            time.sleep(0.1)

    #print('out of loop')
    #time.sleep(10)
    #GPIO.cleanup()
except KeyboardInterrupt:
    GPIO.cleanup()

Reason I expected LOW to turn off LED is because in this diagram, LOW turns off LED

The code is

import time

ledPin = 11    # define ledPin

def setup():
    GPIO.setmode(GPIO.BOARD)       # use PHYSICAL GPIO Numbering
    GPIO.setup(ledPin, GPIO.OUT)   # set the ledPin to OUTPUT mode
    GPIO.output(ledPin, GPIO.LOW)  # make ledPin output LOW level 
    print ('using pin%d'%ledPin)

def loop():
    while True:
        GPIO.output(ledPin, GPIO.HIGH)  # make ledPin output HIGH level to turn on led - Here HIGH TURNS ON LED
        print ('led turned on >>>')     # print information on terminal
        time.sleep(1)                   # Wait for 3 second
        GPIO.output(ledPin, GPIO.LOW)   # make ledPin output LOW level to turn off led
        print ('led turned off <<<')
        time.sleep(1)                   # Wait for 3 second

def destroy():
    GPIO.cleanup()                      # Release all GPIO

What am I missing? Has it got to do something with serial or parallel connections? Why does the first circuit doesn't use GND?

I can see differences in the circuits (eg. 1st one is connect to 3V while second one is not). But I can't figure how internally with RPI, things are getting controlled

Answer 1

Setting an output pin HIGH outputs ~3.3V; LOW ~0V.

Both have limited current sourcing/sinking capability ~16mA.

What happens (and what current flows) depends on the external connections.

In general current potentially will flow out of a HIGH pin and into a LOW pin.

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Incidentally, the way you have wired 3.3V — resistor — LED — GPIO is the way most engineers would connect the circuit.
This requires the GPIO to be LOW to turn the LED ON.
This seems to confuse many beginners.