Source: https://www.instructables.com/id/Automatic-Rubber-Band-Catapult/

Tired of using hand to throw thing against your friend? Grab your tools and build the most powerful automatic catapult in the whole building! Defeat your classmates with this catapult with just a click on the button!

Step 1: Step 1: Prepare Your Supplies!

3D-printer
Screwdriver
Pliers
Hot glue gun
Soldering iron & solder
SawRubber band
M1.4 Bolt and nut
Arduino UNO
2x Micro servo SG90
2x Pushbutton
10k Resistor
Breadboard
Paperclip
Wire
3mm plywood sheet
USB cable
Tape

Step 2: Step 2: Printing the Parts!

3D print the STL files attached. I used the Creality Ender 3 with 1.75mm white PLA.

These are the settings I used:

Infill: 20%
Layer height: 0.2mm
Nozzle temperature: 200°C
Bed temperature: 60°C

The full printing process took about an hour with the above settings. If you don't have a 3D printer you can use cardboard to make it!

catapult.stl
Download View in 3D

Step 3: Step 3: Assembling the Catapult!

Take the bolt and use the screwdriver to put it in the middle hole of the part that looks like a spoon.
Grab the rubber band and secure it around the bolt with the nut.
Use a bit of hot glue if necessary. Use pliers to straighten the paper clip and cut it in half.
Put one half of the paperclip into the holes of both 3D printed parts and bend the ends to make sure everything stays in place

Step 4: Step 4: the Circuit!

Step 5: Step 5: the Code!

Here's the link!

https://create.arduino.cc/editor/kimiho0203/3dde9654-e0ef-43c9-801d-f3db29e78e4a/preview

Step 6: Step 6: the Casing!

For the casing we'll be using 3mm plywood. I cut out 5 pieces with the following dimensions:

8x6 cm (1 piece)
8x5.4 cm (1 piece)
6x12.7 cm (2 pieces)
8x13 cm (1 piece)

Drill a hole in the center of the 8x6 and 8x5.4 piece (make sure it's big enough for the 3 wires of the servo). Drill a hole of 1.1 centimeters in the piece of 8x13 cm as shown in the picture.

The 8x13 cm piece will be the top, the other pieces are the sides. Use the glue gun and glue all the pieces together to make a box.

Step 7: Step 7: Finishing the Catapult!

Put everything in the casing, but make sure the servos and power cable are on the outside. Glue the button to the hole on top of the casing and you're almost done!

Glue the base of the catapult to the top. Make sure there is no tension on the rubber band! Finally glue the lock servo to the other side of the casing. Make sure the servo has an angle of 180° and is blocking the arm of the catapult.