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.