From what i readed on arduino forum(forgot the topic name) i found that a cap will reduce the risk of a high voltage entering arudino
Not really. For DC voltages it will make no difference. For some cases of spikes, it will help. It should not be considered a protection against anything. You will probably buy a 25V type, which means it would be damaged by anything above 25V. It therefore also relies on R1 for protection.
I also used the zener in other reading module(same divider) shoud i unsolder them all ? is there any problem if i use them in term of damaging the arduino or something else ?
I could simple unsolder them if there is a problem.
Without seeing your schematic for the other inputs, I would not recommend changing anything. If it works, then don't change.
- The thing is that i was trying to figure it out, how does the market module for this "signal lighs" works, without a ground wire
Which module? Show a link. Use [ url ] tags on the link. It's not important anyway.
- i already ordered 2 p-channel mosfets that will handle that current (i'll quote you when they arrive) EDIT: IRF9Z34NPBF
Rds = 0.1
Id = -19A
OK, that is more like it. That will probably do as the primary high-side switch. It will need a heatsink if you are passing more than an amp or two through it.
My favourite reference page when I forget if I need an N or P mosfet is this one: MOSFET as a Switch
So to use that MOSFET, you need to raise its gate voltage to be equal to or greater than the source voltage to turn it off. Since source will be connected to 12V in, this means you must have a way of holding this gate at 12V. To turn it on, you need at least -10V between the gate and source (the negative means the gate voltage is lower than the source.) That part is easy. Getting the high voltage will require more components as the Arduino can't output any more than 5V.
Usually you would use an N-type MOSFET - configured as a low-side switch - to switch the voltage on the P-type's gate. A pullup resistor will provide the high voltage.
A much easier alternative is to use a packaged high-side switch. My favourite is the BTS716G. It is designed for automotive switching and has a lot of features that make it very reliable and safe. You can drive it directly with an Arduino and it doesn't need a heatsink to switch very large loads.
- I tought if i won't use those diodes, the current will flow back to the relays or something, like a protection witch i'm not sure about
Depending on the rest of the car's electronics, feeding voltage back up the switch wire may or may not be a problem. The diode is a good insurance if you're not sure.