Hi,
I have just one question:
Is it possible to detect of there is power on a wire?
I want to make something with car diagnostics, like when the doors of my cars are opened etc.
So it this is possible, I can just wire the arduino to the light indicator lines of my dashboard.
So I want to detect if the door light is turned on (there is power on the line to the indicator light) and detect it in my Arduino.
I want to do this with more indicator lights.
Any suggestions? Thanks.
I think what you want to do is to detect if current is flowing rather than there is power on a wire. With a light switched off there is power on the wire but nothing is flowing.
Anyway the simplest way is to use an opto isolator across the load you want to monitor. Opto-isolator - Wikipedia
This looks like an LED so you need a series resistor as well. Wire this across your load and wire the output transistor to your arduino. Emitter to ground collector to input, and enable the internal pull up resistors.
Presumably you could also use a transistor from the power line.
Mowcius
yes but then you have to worry about common grounds and all that, it can get messy. So I think an opto is probably best.
Oh yeah forgot about the common ground issue 
Opto-isolator all the way
Mowcius
Do you have an example diagram? I have never used a Opto-isolator before..
Sorry, never used one. retrolefty (online) or grumpy_mike (offline) will know
Mowcius
Sorry forgot to include the link:-