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I've gotten contradicting information on the subject. Some say that digital input pins send out positive voltage, and if it goes to ground (for example) it reads LOW but if the pin is connected to power, it will read HIGH. On the other hand, some say that the input pin is receiving voltage from either power (+5V) or ground (0V). which one is it?

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Digital pins sink or source current. They don't provide negative voltages. If the pin is set to INPUT then it is high impedance and will not "send out positive voltage".

However if you activate the internal pull-ups, as Ignacio said, then effectively the pin is connected to +5V via (approximately) a 50k resistor.

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It's both, depending on whether or not the internal pullup has been enabled (INPUT versus INPUT_PULLUP). The pullup ties the input to the positive supply through a moderately large resistance, meaning that it is possible to source a small amount of current from the input pin.

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