gpio value zero turns the circuit on why?

Question

My Pi is acting in a way i'm not expecting. When I set the value of a pin to 0 like

'echo "0" > /sys/class/gpio/gpio18/value'
or when using python
GPIO.output(18, GPIO.LOW)

it has the effect of turning on the LED I have wired up. Also when I set the value to 1 it turns the LED off.

Is my Pi broken or am I missing something?

it is connected to the 3.3v power with a 270 ohm resistor followed by the LED then a connection back to GPIO18 pin — Robert, Aug 21, 2013 at 5:33

lenik · Accepted Answer · 2013-08-20 22:19:05Z

7

that's correct behaviour, if your LED is connected to the +5V rail with a resistor:

GPIO resistor LED     +5V
   o---////---|<|-----x

you may try to connect it to the GND (ground) if you like to reverse the ON/OFF depending on the value you set to a GPIO pin.

answered Aug 20, 2013 at 22:19

lenik

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2

So if I understand correctly by setting the pin to zero it has the effect of being a grounding pin and completing the circuit, is that the case?
– Robert
Aug 21, 2013 at 5:42
@Robert : you're correct.
– lenik
Aug 21, 2013 at 8:46

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