This is a machine whose sole purpose is to crush a banana and launch it.
It was built by Benjamin Ojanne and David Törnqvist at Tullinge gymnasium in Stockholm.

Step 1: The Box/frame

You need four wooden boards.
First two wood boards need to have following measures: 20 cm * 10 cm. The thirth wood board need to have following measures: 25 cm * 20 cm. The last wood board need to have following measures: 20 cm * 10 cm * 3 cm.

Start with glue together the first 3 wooden boards. Then glue together the last board at the bottom.

Okay when the box is done, you need to make some holes. The first two holes you need to make is at the bottom of the box on the backside. These two holes need to match with the latch. They holes need to be 12 cm in diameter and need to be placed 5 cm from the side. Otherwise the lock latch will not match with the holes, because we want the sticks to go through the holes to lock the catapult.

It's pretty easy to do. Take help of the pictures in step 7 and try to find out where all the holes should be.

Step 2: The Launchpad

When building the launchpad it is important to consider the size of the banana that you are going to use. Basically you want your launchpad to big enough to house a banana plus som marginal for the springs. If you are unsure 24 by 6 centimeters should suffice. Consider using a lighter material for the launchpad as you want it to flip up fast and launch the banana. Balsa wood is a great choice.

Cut out the launchpad and drill two small holes in both upper corners. Then thread the springs through the holes and glue them in for reinforcement.

Flip the launchpad upside down and glue a straw alongside the bottom, opposite from the springs. consider using a very strong glue for this as a lot of force will be exerted onto the straw.

It is a good idea to reinforce the straw’s attachment to the launchpad by drilling holes alongside the straw and threading metal wires through the holes and around the straw. We used paper clips for that.

For last drill two holes next to the springs rougly 4 milimiters in size. These will be used later for the winding device.

Step 3: The Winding Device

This step is completely optional. Instead you can opt to manually arm the launchpad.
First you want to get a shaft. It should be quite sturdy as a lot of pressure is going to be exerted on it. We used a wooden rod with a diameter of 2 centimeters.

Next you’ll want to make the supports for the rod. You need two bits of plywood roughly 5 centimeter wide and 6 high. Then drill a hole in the upper part as shown in the picture above. It is important to make the holes big enough for the rod to rotate freely and align the holes so that the rod can be straight to reduce friction.

Afterwards you’ll want to glue the supports to the bottom of the box. Make sure to use a strong glue. The supports should be on the opposite side of the launchpad attachment.

Drill two holes roughly 4 millimeters in size in the shaft. They should be big enough to tread some string though but don’t make it to big or you’ll risk breaking the shaft. The holes in the shaft should align with the holes at the bottom of the box.

Thread a string though the rod and through the base of the box and through the holes in the launchpad. Then back through the hole in the base and make a knot with the other end. Repeat on the other side so that you can rotate the shaft and pull down the launchpad

Finally put a crank or a motor at the end of the shaft.

Step 4: The Latch

To build the latch that lock the launchpad, you need to have two sticks with diameter around 2-3 cm. The length need to be only 5 cm.
Then you need to saw out one wooden plate that is 25 cm * 3 cm. You need also a drilling machine with a drill with diameter of 12mm.

Step 1: Saw out two sticks with the correct measures.

Step 2: Saw out the wooden plate with the correct measures.

Step 3: Drill two holes on each seat of the wooden plate which has a diameter of 12mm. The holes must be 5 cm from each side.

Step 4: Glue the sticks to wooden plate.

Step 5: Instead of the piece of wood in the middle you need to glue a water spray up and down. This task should be to push in the latch and then pull it out to shoot away the banana .

Btw On the image we have a piece of wood in the middle but that is something that YOU DON’T need!

Step 5: The Press

To start off you’ll want to drill 4 holes in a wooden board. They should be the size of your syringes, ours had a diameter of 20 millimeters. The spacing between the syringes should be narrow enough so that the pressing plate can fit in between the springs for the launchpad. They should also be far enough from the back wall to not interfere with the latch. Next you’ll want to glue the syringes into the holes. Regular hot-glue should suffice.

Afterwards you want to make the pressing plate. It should be of a size such that the syringes attach in every corner. Our measurements is 7,5 by 15,5 centimeters.

Finally you want to connect all the tubes. You will need to split one tube into four. We achieved this by using three two-way splitters and connect the ends to the four syringes. It can be helpful to heat the tubes with a heat gun to make them expand slightly. But be careful as if the tubes expand to much they begin to leak and when the pump is going to suck out the water it sucks air instead.

Step 6: Pump Control Logic

This is basically our circuit with few exceptions.
-The motor is actually a 12 volt pump.

-we used the l293d ic as a h-bridge instead.

Our specs:

-12 v battery.

-The 4 resistors at the top is 1kΩ to bring the current down to 5v for the l293D

-The pull down and the resistors connecting the push buttons is10kΩ

Step 7: Putting Everything Together

First attach the launchpad by putting a floral stick through the side of the box, through the straw and out on the other side. Glue it in. For the launchpad, glue the springs to the screws on the inside of the box such that it forms a catapult of sorts.

Take a 5 by 9 centimeters wooden board and make a 2 centimeter hole in it. Next put the latch in the holes at the bottom of the backside of the box. Glue the pushing end of the syringe to the middle of the latch. Screw in the wooden board and glue the syringe in place to complete the latch mechanism.

Put the press on top of the launchpad and make sure it is almost fully extended and screw it in from the sides. Connect the tubes from the latch and press to the outputs of the t-valve. Connect the pump's output to the input of the t-valve. The input of the pump you’ll want to put down into a water tank. We used a water bottle glued to the back for that.

If you want you could also glue the pump logic and batteries to the board beside the latch. Another cool thing would be to connect everything to a logic circuit or ardurino so that all the operations would be automatic.

By Benjamin Ojanne and David Törnqvist