Somebody on this site made a mine like this but i modded it so now when someone opens the door they get a shower of spacers.PRSAC (please rate, subscribe, and comment)

Step 1: The Base

This is the base.

Step 2: The Motor, Part 1

You have to have a spring motor to build this.

Step 3: The Motor, Part 2

This is where you add the motor and shrapnel.

Step 4: Setup

Make sure that the orange rod is in so that it will not fall out.

Step 5: How It Works

So here's the gist of it: Victim opens door, rod falls out and things go boom. I also recommend setting it up on a high dresser or something and balancing other knex grenades on it so that when the mine is activated, they fall to the floor and blow up.