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If I do not specify the output of GPIO pin (like GPIO.output(4, False)), I always have a voltage of 1.3V on this pin

How can I set all pins to 0v? and why are undefined pins have this voltage of 1.3v???

2

The default state of pins is given in http://www.panu.it/raspberry/

The default Device Tree settings configure most pins as inputs, some with pullup others with pulldown.

Pins set as input cannot be said to "have a voltage", except for the high value pullup resistors they are in a high impedance state. Any voltage you measure is a artefact, and depends as much on the measurement circuit as the state of the pin.

If you want to change the pullup on a input pin you would do something like GPIO.setup(4, GPIO.IN, pull_up_down=GPIO.PUD_DOWN)

Incidentally DO NOT use constructs like GPIO.output(4, False) - You are NOT setting the pin to False, use 0 or 1 (or constants like GPIO.HIGH) which will make your code easier for you, and others, to read.

0

With the pin defaulting to be an input, and with no pullup/pulldown defined you are just looking at the (relatively undefined) high-impedence voltage of the input pin.

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