Short-Circuit prevention

Other Hardware General Electronics
1

Hello,

I am a little bit worried about causing a short circuit while using one of the digital output pins. According to this tutorial: http://arduino.cc/en/tutorial/button, the button diverts current from the ground pin to the digital input pin and when that occurs the reading on the pin is HIGH. The current comes from the regulated 5v pin which, when the arduino is powered via usb, produces between 100mA to 500mA. Wouldn't that current damage the digital input pin if connected directly? Also, I have a 9V, 1A external adapter, would using that adapter damage the digital input pin as well?

2

You only apply 0 volts or 5 volts to a digital input pin.
If you follow this rule you be OK.
That tutorial follows the rule.

You never attach 0 volts or 5 volts to a digital output pin directly.

3

The 10 K resistor keeps it from shorting to gnd and a pin set to input uses little power but I always put a 1 K on the input pin just to make sure I dont set a pin to output and set it low.

4

The 10 K resistor keeps it from shorting to gnd and a pin set to input uses little power but I always put a 1 K on the input pin just to make sure I dont set a pin to output and set it low.

That's a good idea to use 1k, at least until you build a standalone circuit. Here I used 240R

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5

So the current draw from the input pin is never too high, thus never requiring a resistor? the 1k resistor thing is a good idea by the way, thank you.

6

Assuming the microcontroller is running from a 5V supply, you can apply any voltage
between 0V and 5V to an input pin safely. It will take no current at all (almost - less
than a nanoamp).

If the voltage isn't near 0V or near 5V it may read randomly, the signal's meant to
be either HIGH or LOW, not inbetween! You may also see the chip's power consumption
rise if an input is somewhere in the middle, unless the input has hysteresis built-in.

If you supply less than -0.3V or +5.3V then the input protection diodes will start to
conduct and the current can rise - more then a mA or two may see damage occur
(these diodes protect from mild electrostatic discharge, not continuous current).

If the pin gets set to an output and you have it connected to 0V or 5V directly, large
currents can flow (100mA or more) which will damage the pin's output buffers,
the allowable limit is 40mA ("absolute maximum").

So adding a 1k resistor in line with the pin will prevent such damage by mistake,
which is useful if constantly updating different sketchs with different pinMode()
settings. If driving something that needs a decent amount of current a 150 ohm
resistor is good - prevents reaching 40mA, but allows LEDs to light up brightly.