This is an extremely simple Knex Trap. It is inspired by https://youtu.be/n4cYERKlGKU
You can also use anything as an alarm for this trap too. Im using a popping toy thing.
Note: Use fishing line, im using yarn for demonstration purposes.

Step 1: Parts List

This is a very simple build, which shouldn't require much parts.

Knex pieces:
16 Yellow Connectors
8 Red Connectors
10 Blue Rods
2 White/Black rods
1 Orange Connector
16 Yellow rods
2 Large Plate pieces
2 Blue Spacers

Non Knex Parts:
2 Loom Bands ( small elastic bands )
Weight ( anything heavy enough to make the model to stop sliding once triggered )
Anything that makes an alarming sound when force is applied ( snappers, popper )
String/Yarn/Fishing Line ( about a few more inches longer than your doorway )

Step 2: Building the Mechinism

1. Make the frame as shown

2. Set the previous model aside and add 2 spacer on a blue rod, along with an orange connector with a yellow rod and a yellow connector.

3. Add yellow connectors to the blue rod as shown.

4. Attach that model to the 2 frames as shown.

5. Add an elastic band as shown.

Step 3: Building the Areas Where the Weight and the Alarm Will Go.

1. Build the basic frame as shown. ( ignore the knot of the red connector of the bottom left corner.

2. Attack the previous model to it.

Step 4: Building the Other Side of the Trap

1. Tie your string onto 2 blue rods, 1 rod with a red connector as shown.

2. Build the frame, similar the one at the begging as shown.

3. Build the square as shown

4. Connect them all together.

Step 5: Setting Up Your Trap.

First add dumbells or any weight on the black square.
( you might want to raise up this trap on some books or mats so it has a higher chance that your victim will trigger it )

Then, set one of them on one side of the door way and the other model on the other side. ( im using my garage door, lol )

Step 6: Setting Up Your Trap ( 2 )

Bring the yellow rod and connector part on the other side, then put the arrow looking part between the black rods and the yellow rod, making the arrow stuck, unless it has been triggered.

Step 7: Making Your Alarm More Effective

In the empty square frame, go ahead and put in something that can make a loud noise once a heavy force has hit it.
Im using a popper. You can go ahead and use snappers, or rig up a phone to it so it can dial you once the trap has been triggered!