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[Reply #15 on:]

Ok, thank you. I´ll draw a schematic and post it here, to prohibit errors on my part

[Reply #16 on:]

So, does this seem about right ? I don´t understand completely how the NPN works, because I thought it had to have a common GND with the Arduino and the car.

[Reply #17 on:]

Yes, you need to connect Arduino ground to the car ground (i.e. negative side of the 12v battery) as well. If you're running your arduino from the 12v car supply, you must be doing this already.

Don't forget the diode across the relay coil.

[Reply #18 on:]

Yeah, It´s all connected to the car battery. Then i have a straight-forward way of doing things! Thank you all so much, love this community!

I have some 2N3904 (NPN´s) and some 1N4148 diode´s on hand, so I´ll just use that.

I have also done a a test run on the Arduino and the clicking from the SPDT seems much "firmer" now, so I´m quite confident

[Reply #19 on:]

This is actually working and turning out awesome. Thanks a bunch!

[Reply #20 on:]

So, after hooking this up to the vehicle something strange is happening time to time. Sometimes there is very little light from one of the relays, sometimes there's more. It's the same circuit that's acting up. What happened just now was that the circuit continued to give out power even after the Arduino was shut down. How is that possible?

[Reply #21 on:]

Sometimes there is very little light from one of the relays, sometimes there's more.

I'd be very concerned about ANY light from any of the relays. Sniff. Sniff. What's that burning?

[Reply #22 on:]

No, no, bad choice of words. Of course I mean output. Current. Voltage. Electricity

[Reply #23 on:]

If the relay isn't passing the full current, then it sounds like you have a dodgy connection or a faulty relay. If it stays on when it should be off, that could mean a faulty relay, shorted transistor, or shorted wiring. Do you get a good clicking sound from that relay when you turn it on and off?

[Reply #24 on:]

So, after investigating a little, I decided to change the resistor to a 1kO on just that circuit. The other ones run a 0,8kO resistor. So far everything is running smooth. If there is a error, I think it's the transistor and will change that as soon as I get one.

[Reply #25 on:]

I know its been a while since I started this project, its actually a while since I finished it!

Thank you all for your grand help!

You can see (some) of the result here:

http://youtu.be/LkfvAbjPgRU