How will I go about making a momentary switch with a relay?

I'm making plans to build a carputer (computer in a car) for my car sound system, i've found a pc power supply suited for the purpose, problem is, it's not a more expensive one that was made for the purpose, so i'm using relays to control the on/off of it, and i don't want a sudden off on the... show more I'm making plans to build a carputer (computer in a car) for my car sound system, i've found a pc power supply suited for the purpose, problem is, it's not a more expensive one that was made for the purpose, so i'm using relays to control the on/off of it, and i don't want a sudden off on the computer.
You may be asking, why don't I just get a power inverter? cause that will be to easy, i'm wanting to do this as a challenge.

The psu i've found...
http://www.mini-box.com/picoPSU-160-XT

The psu meant for the purpose...
http://www.mini-box.com/M3-ATX-DC-DC-ATX-Automotive-Computer-car-PC-Power-Supply?sc=8&category=981

So, as you can see with the car psu, I get less wattage for more price where I can make a 2 or 3 relay module for just a few dollars. The reason for these relays is because, in a pc psu, there is a constant 5 volt rail that keeps the RAM stable until next turn on, well that will drain the battery of the car, so I need the relay to completely cut power from the psu when the car is turned off, but if i forget to turn off the pc first, it will be a sudden off which is not good for a pc and i don't want to worry about that, so i'm wanting to make a momentary switch with a relay to pulse the off signal to to the board so it turns off before the psu is cut off and to pulse on when the car is on.
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