I've been doing a lot of reading on using opto isolators in the noisy automotive environment but I keep coming back to the same issue in my head...
In order to completely isolate both sides of the isolator you can not share the grounds. So how is this accomplished in a car?
If you're going to power your arduino from the car battery then you can't. So the opto-issolator solution is pointless.
Ok thank you. That's what I was thinking, but I couldn't find anything solid that confirmed that.
UNLESS, you create an oscillator to drive a transformer then rectify the output of that to and derive your power for the arduino. This is probably overkill for your application though.
Yeah that would be, i'm only turning on and off 3 solenoids which draw around 2A and then a 7 segment led. I'm building everything to handle 10A, but i'm sure i have taken some decent steps towards protecting the arduino.
Hi,
Post us a copy of your circuit, either CAD or a picture in jpg, png, or pdf of a hand drawn schematic will do.
Tom.... 
TomGeorge:
Post us a copy of your circuit, either CAD or a picture in jpg, png, or pdf of a hand drawn schematic will do.
I get the impression that the OP hasn't got as far as working out a schematic. Just a hint of what he wants to read would be helpful though.
I am still working on the schematic, but basically i'm trying to drive a P-channel mosfet to trigger a solenoid in a transmission. it has to be a HIGH side driver because i can't get to the ground of the solenoid.
This is the driver circuit that i've come up with so far:
Sorry i don't know how to post the schematic directly 
the opto isolator should be where the switch is in the schematic.
Just so it you have as much info as i can, I'm also using one of these cheap buck converters from ebay to regulate the voltage to 5v for the board.
I'm sorry if this doesn't make much sense, i have parts laying all over my desk and nothing more than a copied code and chicken scratches for the design.
I don't see any need for an opto isolator.
Alright, that simplifies the design too.
My naturally curious mind now asks... At what point are the grounds considered "separated"? Take for example my desk, for now the Arduino is powered from my computer, and the 12v side is powered from a benchtop power supply. but both are plugged into the same 120v electrical outlet. I know thats a stretch, but short of a battery they are never really fully separated. Or i am just over thinking it?
Dingo509:
Alright, that simplifies the design too.My naturally curious mind now asks... At what point are the grounds considered "separated"? Take for example my desk, for now the Arduino is powered from my computer, and the 12v side is powered from a benchtop power supply. but both are plugged into the same 120v electrical outlet. I know thats a stretch, but short of a battery they are never really fully separated. Or i am just over thinking it?
Once the power has been through a transformer it has been separated. Your computer and the benchtop power supply outputs are separate.
Ah! Ok that makes sense. Thank you all for explaining this too me.
It's not so simple.
The problem you may face with a common ground in an automotive situation, is that you don''t actually have a direct connection to the chassis. If you are passing a heavy load current through a wire between the Arduino module and the chassis, then a load current will generate a voltage drop and transients across that wire and if you are trying to assess other external signals, particularly from analog sensors, then that voltage will be impressed (negatively) on the signal.
So you will want separate ground connections from the Arduino circuitry to ground, and from any control device such as a transistor or FET that is switching a significant load, to ground. That is where you may well want an opto-isolator.
Hi, if you are going to be controlling inductive DC devices in a car, may I suggest you use a purpose built automative high side switch.
These devices are logic level driven and provide protection for the majority of faults that can occur in an automobile environment.
Tom............