Newbie Question - Car Alarm

I am completely new to Arduino, I did some small stuff with Picaxe a long time ago as part of a class. I really want to learn how to do some of this and understand it fully. For my first big project, I’d like to design a car alarm (I know I can buy one cheap, but where’s the fun in that?). So, I’m just looking at some simple inputs and outputs. If the car door opens and after so long a period of time someone doesn’t turn on the ignition then a light blinks or a relays closes, something along those lines.

I’m wondering if anyone has some example circuits combined with some lines of code that I could look at to serve as a guide? I’ve looked at some of the tutorials and see some simple switches. Here’s one question I have right off the bat. Most of the simple switches I see, the switching is done at the 5 volt level. If I isolate the 12 volts from the car away from the 5 volts at the Arduino, but have them share a common ground, can I do the switching at ground instead of the 5 volts? My example being that most car door switches go to ground when the car door is opened, so I would want the Arduino to sense that switch is going to ground. Anyone already have a circuit drawn that would show how to do this? I’ve done some forum searching and haven’t came up with anything.

I appreciate any help you can send my way!
Thanks for such a great forum.

As a former car audio and electronics installer, I’ll help you out a bit. You’d be wise to not force the system to rely on a negative door trigger but leave it changeable via a hardware change, or do what DEI does and have both a positive and negative door trigger lead.

As far as relay switching, there’s a couple of examples of how to do this using a transistor and blocking diode.

I appreciate the reply, and you are right, the option of two different inputs, both positive as well as negative is a good idea. Can someone give me some guidence and direction on how to go about this? I’ve seen the diagrams of using diodes and transistors to control relays, I fully understand that and shouldn’t have any problems there. It’s the inputs that concern me.