Timed relay, how to?

I want to build a setup that when when you send a high amperage momentary signal to it, it activates a relay for about 30 seconds allowing the starter circuit to be completed.  The signal will come from my remote for the door actuators.  it has built in relays, so i have to use that high amp signal to activate my "box" that will allow the starter circuit to be completed when the key is engaged.  when this box is not activated by the door, it will send a signal to a siren when the key is turned...why am i building this? Im bored and want to build something creative.  I know i need a relay with a cap in parallel to it for the time delay, but how do i figure out how big a cap i need, and how do i drop the amperage down from my doors to keep from frying the 1amp diodes i have?

I hope I explained that correctly!  I do not want to order anything...i want to build this from scratch.

you wont be able to use just a capacitor, unless you get a giant cap (greater than a farad).

this should work, the simpliest I could whip up... you could use a 555 in monostable mode as well:

thanks for the quick reply...i have a basic understanding of electronics, but i dont completly know the ins and outs of transisitors...also, where do i get that transistor?  what does D1N4001 stand for? is that the reference number for the diode?  is that the one amp diode i mentioned?  what is the purpose of D2?

it makes sense, but it doesnt!  this is the only way to learn and i appreciate your help!

what is a 555?

The transistor i used for that was random, if you try it just use any NPN darlington, most electronics shops have darlingtons... or you can even just go to radio shack and get a standard NPN (like a 2n2222) and connect them as a darlington.

the purpose of D2 is the kickback diode on the relay coil see: https://www.the12volt.com/installbay/forum_posts.asp?tid=28260&get=last

1n4001 is just the type of diode, a very common one...