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I have a chinese arduino nano that I'm planning to use to open a door using a numeric keypad.
My power source is 12V and I want it running 24/7. I have read that the first thing to fail on an arduino is the voltage regulator due to the heat of lowering the voltage to something the arduino can use.
I have think of buying two big resistors of around 1W each to build a voltage divider and only feed 7V to the Vin pin of the arduino, so it will increase its lifespan.
Is this a good idea?
Terminating voltage by dropping it over large power resistors is a bad idea, efficiency wise. There are better options.
Switch-mode Buck (=step down) converters are usually far more efficient than linear regulators. A linear regulator dumps the excess voltage in a resistor which converts the energy mainly into heat. A switch-mode converter works by switching on/off an inductance at the right time, which is far more efficient.
For step-up/step-down DC-DC converter theory look here, here and here. For a comparison for switch-mode against linear regulators look here.
A step-down converter module like the ones with a LM2596 should be good enough for you. See here on ebay.
