I accidentally gave 12V to my Arduino Nano's 4th digital pin. It was coded to stay HIGH from the setup. Would that break or harm anything?
All depends on whether you gave 12 volts negative or 12 volts positive AND where the other half of the 12 volts was connected. Did it make a complete circuit so 12 volt current went through part of the Arduino circuit?
I gave +12 from the same source Arduino was using. Nothing visible happened such as sparks or fumes but it got me worried
Over-voltage protection circuits. I'd Increase the series resistor to 1K or more in an application where the signal will always be 12V. If you are occasionally-accidentally going over 5V, the 100-Ohm resistor should be fine.
Does the Arduino still run? Does that input work?
The specs say a maximum of 1/2V volt over Vcc (typically 5.5V) and or 1/2V negative.
There are actually "small" (low current) protection diodes built-into the ATmega chip. (They are primarily to provide some static discharge protection.) With a 10K "current limiting" resistor in series with the input, those internal diodes will "replace" the diodes in the protection circuit. But if you directly-connect a 12V power supply with no resistor, you'll fry those internal diodes and probably fry the whole chip!
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