That's what that resistor is for - to protect the pins. It's usually not voltage itself that does the damage but the resulting current (that is, until voltages get so high you get sparking - but then you're talking about static electricity with voltages of possibly tens of kV).
Look up "clamping diodes". In the data sheet (of the Mega328 at least) is described how to use them to safely read high voltage inputs. There are even circuits out there that use the clamping diodes to do zero crossing detection of 220V AC mains. Wouldn't do it myself (due to the high voltage that's not isolated), but the processor won't be damaged.
Well, I have to say that my car is very well acoustic isolated from the outside, you almost do not hear the engine noise running (and it is a diesel engine!).
The only "clicky" sound I hear is from the turn lights and I have to say that the clicky is soft, it almost appears to be a digital feedback.
Now, the relay module I bought is quite noisy and it is rated to 15A/24VDC, maybe others don't do so much noise, I don't know.
When I set it to turn on and off, I can hear its clicky from another room in my house, it will probably be quite noticeable inside the car then.
I would make R1 100k and R4 47k.
That will still give a 'high' to the pin (>VCC*0.6) when battery voltage drops to ~10volt (during starting).
The current values won't.
And, as said, double-up the width of those source/drain traces.
You can reduce footprint of this circuit to 1/4 of the size if you want.
Post the Fritzing file if you don't know how.
Leo..
If there is room, make your pads 50mil or larger.
80mil for terminals, wires, TO220 etc.
You will thank yourself if you make your traces wide, 30-50mil if possible.
For 0 volts, use a ground plain.
Not sure how to make a board outline and move the traces on there in Fritzing (I use Eaglecad).
Then you can make the board a fixed size, like 25*30mm.
And flood-fill the open spaces with a ground plane.
If you do this in a standard cad program, then you can order a bunch of these small boards panelized for US$5+freight.
Fritzing (no Gerber files?) seems to be a dead-end street.
Leo..
Ask yourself, once the holes are drilled at the size you designed for each component lead, will there be enough copper around these plated holes.
If not, increase the pad size.
One last time, use a ground plain, GND pads and vias with thermal relief.
Gerber files are what is needed by the PCB company to manufacture the board.
Top foil, Bottom foil, Drill file, solder mask and silk screen.
Yes, but name the floodfill "ground" (or set the ground via to GND fill seed???).
Then is will be connected to ground, eleminating the ground traces.
Also make the board outline a bit wider, so the gap closes around the three outer vias.
The five connection vias seem very small. Are you going to use a connector or wires.
Can change the properties of the via to e.g.
hole: 0.9mm and ring: 0.5mm.
Leo..
Will you be needing holes for standoffs or screws (mounting holes) ?
Add pads for, a LED/and series resistor, placed in parallel with the load. You don’t have to populate these but at least they are there if you change your mind later.
Gerber files will be need if a PCB house will be making the board.
