Introduction: Motorised Trebuchet Controlled by Arduino Uno

About: HKU Student Games and Interaction

How to make a motorised medieval trebuchet that can be controlled by an Arduino.

I'm going to show you how to build a motorised trebuchet with an Arduino Uno.

I'll be assuming you have the necessary tools and skills to follow this guide.

Before we start you'll need a couple of items:

  • Arduino Uno
  • Breadboard/PCB
  • DC 4,5v-9v motor
  • Jumper Wires
  • A push button
  • MOSFET Transistor
  • A generic 1 A 30 V Schottky diode
  • A 10K resistor
  • 9v Battery
  • 2 a 3 Small Square Wooden Bars
  • Round wood
  • Wooden Scrap or Something you can saw little pieces out with a thickness of at least 1cm.
  • Wood Glue
  • Small piece of Leather
  • Small Rope
  • a pop nail
  • A piece of Valve Hose
  • 2 Iron clips
  • Pickle
  • Wide Elastic Band
  • Long nail
  • 2 small hooks

Most materials are super cheap so first prototype before making a final product.

Step 1: Building the Framework

In this step we'll be taking a look into thinking of a basic design and how to start building the frame for that design.


First, I like to draw a concept/basic idea of what I want my trebuchet to look like.

When you've come up with an idea you like. We can start building the frames.

Building a frame

I sawed a long squared bar in 8 pieces 4 of equal length, 2 somewhat longer pieces and 2 crossbars. (Sizes vary from how big you want it to be.)

I made 2 notches in the crossbars deciding the width of the frame, in the 2 notches I put the 2 longer bars, deciding its length. This is the base of the frame. DO NOT GLUE YET,you might want to make changes later on. I made 2 angled notches in both the longer bars, and placed the 4 bars in these notches so it form a triangles.

You can add little diagonal bars for decoration, but those aren't required.

It's important that you don't glue it all together as we have yet to place the axle.

Step 2: Placing the Axle

In this step I'll show you how you can make and attach the axle and how to assemble the entire thing together. It's harder than you would think.


For our swinging arm we're going to need a an axle, one which we'll later on connect with a belt drive.

First, if all fits your design well, go ahead and glue one side of the bars which form the triangle to the long bar, and glue the crossbars as well. It's important you don't glue both sides yet, otherwise we cant place the axle.

Second, saw 2 small triangles that will fit underneath the frame's triangle top.

Place and glue those underneath. Drill a small hole in the side of one of the triangles and drill a hole through the other (This will determine on which side the motor will go.). Make sure the hole you drill all the way through is slightly bigger then the long nail.

Creating the axle

Third, saw a piece of one of the bars just a little shorter then the frame.

Grab your pop nail en place it in the other hole. Now drill a small hole (Once again watch the width of the nail.) in the middle of both sides of the axle (If you fail to centre it, saw of another piece of bar.)

Assembling the axle

You should now be able to place one side of axle on to the already glued on triangle. Grab the long nail and place the other side of the frame on with the screw in the axle bar. The long nail should be sticking out a bit.

Now saw 2 little pieces of the round piece of wood and drill a hole in the side. Place one over the nail sticking out.and cut a piece of valve hose and place it over the piece of round wood. This will be one of the gears of the belt drive. The other one will be for the motor later on in this guide.

Step 3: Adding the Throw Arm and Adding Supports

In this step I'll explain how you can easily make and attach your throw arm and what kind of supports you might be able to add to your design to make it stronger and better looking.

Adding the arm

Saw of a piece of wooden bar about the length of the frame. Make a notch where the axle is. (Or adjust it to your designs.) Also make a notch in the axle and glue them together. If everything went okay, you should have a basic trebuchet.

Adding supports

You can also add in crossbars as supports between the crossbar, these can make sure your project is sturdy enough. In this step I also added a little platform where the DC motor will rest on.

Step 4: Creating the Counterweight

In this step I'll show you one of many ways to make a counterweight. This can be done in many ways but I chose this one as it's modular so you can ease the load on the motor and change the reach.

Modular Counterweights

There are many ways you can do this. I chose a modular counterweight depending on the projectile I'm trying to launch. To do this I took some scrap wood and glued it together with a square hole in the centre, so you can change the balance. For a finishing touch I glued a little thinner plate of wooden the top and bottom so it almost looks like a little crate.

Step 5: Polishing

In this step I'll tell you what you can do to polish your design and make it look wonderful. You can see me build a little stage as well to put the electronics in.

Adding Accessories

If you want any decoration on it now is the time to add more supports. Or wooden accessories. I had added some metal to cover up the rough ends at the top.

Pickle and Varnish

I've sanded the rough edges of my work and I finished it with pickle and varnished the wood. It gives it a more authentic look in my opinion, you can do with it whatever you want to do.

Step 6: Making a Sling

In this step I'll show you how you can make a sling. There are many variations and sizes you can make depending on your previous choices.

Creating the sling

Of course a trebuchet can't fire without a sling. So in order to make one we'll need some leather. Think of the form you want your sling to have and than cut out that form but a little bigger. We want our projectile to rest in the sling, so we make the sling a little shallow.

Once you've created the sling place 2 hooks on the throwing arm and bind two equally long pieces of rope to them. Now puncture 2 little holes in your sling and bind the rope between those little holes.

Voila, we have added a sling to the trebuchet.


However, the sling might not work properly so adjust the rope and sling to give the desired result Note: You can test it out by simply pulling softly on the band drive it should sling forward, but if the projectile doesn't fly right. You have to adjust it.

Step 7: The Electronics

Here is the schematic you'll need to get your dc motor running.

Step 8: The Code

In this step we take a look at the code of our project. We want to make sure the motor only activates with the switch.

Explaining the code

So what we basically do, is set the 5v current from the arduino as our button input. And set our 9v current as output for the motor.

This is the code I used:

//named constants for the switch and motor pins
const int switchPin = 2; //the number of the switch pin const

int motorPin = 9; //the number of the motor pin

int switchState = 0; //variable for reading the switch's status

void setup(){

//initialize the motor pin as an output:

pinMode(motorPin, OUTPUT);

//initialize the switch pin as an input:

pinMode(switchPin, INPUT); }

void loop(){

//read the state of the switch value:

switchState = digitalRead(switchPin);

//checks if the switch is pressed.


//turn motor on:

digitalWrite(motorPin, HIGH); }


//turn motor off:

digitalWrite(motorPin, LOW); } }

Step 9: Shoot!

If you've made it this far you've build yourself a motorised trebuchet, you can continue to tinker with the electronics and code to do other awesome stuff, with your trebuchet. For example make on/off switch or add a speed control so you have inproved targeting.But don't forget to shoot it.......... at your cat for example.