arduino car integration

hello all you wonderfully intutive minds. i am trying to figure out what kind of transistors i need to use for my project. i will be using a uno to control a TFT screen that will be relaying commands to a mega via serial communication to operate all the cool stuff i have added into my car. but the most important thing i want to do is do a pin number lock screen that gives way to a touch screen start ]:) ]:D so im figuring transistors or relays but the arduino can not push a automotive 12v relay so how do i get this to work?

and i am hell bent on this idea so please dont post the "why dont you just do a push button" cause this is purely for the thrill of making it. with that being said who knows what im looking for?

These are available in a wide variety of configurations and available from many sourrces. - Scotty

Look at pighixxx's ABC card number 3. How to connect to a relay. He uses an NPN BC337 transistor and a 2.2K resistor. Also, a 1N4007 diode to protect against the back pulse.

He has a caution that the relay should only be +5 to +12 Volts. I'm not sure why, since the BC337 is rated for much higher than that.

scottyjr: These are available in a wide variety of configurations and available from many sourrces. - Scotty

so using these relays i should be able to just hook straight into the wires off my starter? my main concern is the amperage overloading the relays or 'arking fusing' them together ...any thoguhts on this? i know the starter wire will almost most certainly be high amperage... i know when i had to hot wire a honda a few years ago the starter wire sparked like hell

You asked exactly the right question. And the answer is that those relays are absolutely not going to handle all the current needed to turn over the starter on a cold morning. Those relays say on them that they are rated at 10 amps DC.

However, it might not need to. The little tiny wires that hook up to your key switch wouldn't handle that much current either. I expect there is a solenoid that engages the starter motor, and probably another relay as well, to handle the high current and large gauge wires.

Thanks so much for your insight I knew that about the solinoid but completely forgot it LOL I have a bad problem of breaking things down wrong... dang ADD... So I've ordered a seeedstudio tft screen does anyone have any input on the reliability and or functionality of these screens comments or concerns most appreciated