Yes it's a piano made out of complete cardboard. It is a very simple way of making a musical instrument using only cardboard. And it's modular* too, awesome right?

Cardboard piano is made up of two main parts, a physical cardboard keyboard and computer i.e. processing sketch running in the computer.

This piano uses arduino as its main micro-controller board and processing software for generating tones and displaying GUI of the piano. Whenever You press a key on the physical cardboard piano it is displayed on the computer screen using GUI. Processing uses tone files for generating tones so you can also change tone associated to each key by just changing the tone file. Why not generate drum sounds, sure.

Cardboard piano has 13 keys and one dummy key(only for visual purpose, it doesn't do anything) out of which 8 are white keys and 5 are black keys. I am using octave 4(middle part of the piano) starting from middle c and ends with c of the octave 5.

The tones I am using are C, C#, D, D#, E, F, F#, G, G#, A, A#, B, C.

*Modularity can be achieved at software level using different tone files in processing code not at the hardware level.

Step 1: Material and Tools

Material:

Cardboard
Arduino Uno (or any arduino compatible board)
Aluminium foil
Plastic slide blinder (used in stick file)
jumper wires
A computer (of course)

Tools:

Glue
Hot glue / Heat shrink insulating tube
Polystyrene cutter
Electrical tape
Ruler
Pencil / Pen (For marking purpose only)

Note: You will also need Processing software installed in your computer. Processing is open-source and free so feel free to donate and download from here. You can also watch video series by Daniel Shiffman here for getting started with Processing.

Step 2: Preparing the Cardboard

First step in making piano with cardboard is marking the dimensions on the cardboard. I am providing schematics in PDF file. You can download the PDF file attached for reference.
Use pencil / pen and ruler to mark the schematics on the cardboard accurately.
Now cut each part of the piano from cardboard using polystyrene cutter. Use ruler for guiding cutter in straight line. Please be careful, it is a sharp tool.
Cut the keys carefully because its thin and may curve very easily and your hours of work will go to dustbin.

Base And Encloser Schematics.pdf
Download

Step 3: Preparing the Keys

Keys are very essential part of a piano and it's also true in case of our cardboard piano. A piano key has to return to it's initial position as soon as musician raise his finger from the key. But the keys made out of cardboard does not have this essential property. To make cardboard keys work like an actual piano keys we have to integrate it with some spring material. Here I am using a plastic slide binder from the stick file (shown in the material used section).

Cut the holder in three separate strips.
Cut the plastic strip in size equal to the key size for white keys.
For black keys cut the strip in slightly larger size than the keys.
Paste the plastic strips onto the cardboard keys using glue. Refer images above for better reference about how to paste the strip.
The extended part of the plastic strip will be used to fix the keys on the base.

Note:All operations are done on the back side of the key. Do not do anything on the front side of the key.

Step 4: Make Buttons Out of Cardboard Keys

In this step we are going to give superpower to the simple cardboard keys i.e. we are going to transform a simple cardboard into an electrical switch. How? lets see.

Cut the silver foil in small strips with width slightly smaller than the width of the key and length of about 2 - 2.5 cm.
Paste the silver foil strip on one end of the keys, the end which is free after we attach the key to the base using glue (Use image above for better reference).
After pasting silver foil strips on each keys we will attach wire to the silver foil strip on the key. Strip the end of the wire before attaching. Here I used an electrical tape because soldering is not going to work on the silver foil strip.
Paste the wire around the perimeter or on the back of the key for better wire management and making our piano look neat and clean.
Solder a male connector to the other end of the wire so it becomes easy to connect it with the arduino's female pin.
Use hot glue or heat shrink insulating tube to insulate the soldered joint.

Note:

All operations are done on the back side of the key. Do not do anything on the front side of the key.
Make sure to keep the wire of enough length because it may become hard for each individual wires to reach the arduino easily.

Step 5: Preparing the Base

Base of our piano is important part. It is going to hold all the keys and mountings.

Temporarily arrange all the keys from one end of the base. This step is only to measure that how much area of the base is used to mount the keys. Make sure to provide enough spacing between the keys. Mark the distance between which the keys are arranged.
Cut a 2 cm wide strip of aluminium foil of the length marked in above step on the base.
paste the aluminium foil strip on the base at the lower part, where the free end of the key is going to be placed.
Attach a wire to that strip and solder a male connector to the other end of the wire. Don't forget to insulate the joint. This aluminium strip is going to act as the other end of the button.

Step 6: Assembling the Piano

I hope you have already prepared the piano keys and gave them superpower and also prepared the base. Now it's time to assemble our piano.

Mount the White Keys:

Each key in the piano has a travel distance up to which you can press it. To get this travel distance we need to mount our keys at some elevation from the base. Cut a 2 cm wide strip of cardboard and paste it on the base where you want to mount the keys. Also the length of the strip would be same as the length of the aluminium foil strip we pasted on the base.
Start pasting the "plastic strip" part of the keys from right most part of the base onto the cardboard strip which we pasted in above step.
Make sure to provide sufficient gap between the keys.

Mount the Black Keys:

In a piano the black keys are always at some elevation from the white keys. Again cut a strip of cardboard of the same size as we did while mounting the white keys and paste it onto the first strip. It will also help in firmly mounting the white keys and keep them in their place.
We also have to mount the stoppers for the black keys because black keys are at higher elevation due to this their key travel is large. Also the stopper will act as another part of the button as the aluminium strip did for the white keys.
Cut 5 small strips of cardboard of width 1 cm and length 3 cm. Paste small strips of aluminium foils on each stopper and attach wires to it as we did for all the keys.
Paste all the stoppers in between the white keys where black keys are going to be placed. Solder the wires coming out of each stopper together and solder a male connector with it.
Its time to mount the black keys. Place the keys on the cardboard strip and paste it using glue.

It's done now. We have successfully mounted all the keys.

Note: Arrange all the wires carefully because it may be mess.

Step 7: Connect Keys to Arduino

In our cardboard piano we have 13 keys and one dummy key(which is just for visual purpose and won't do anything). each key will act as a button, so we have to interface 13 buttons to the arduino.

Here I am listing which key to connect to which pin of the arduino. You can change it according to your requirement. Also refer the image for key names(ex. C, C#, D etc).

C(left) - 12
C# - 13
D - A5
D# - 5
E - 6
F - 7
F# - 8
G - 9
G# - 10
A - 11
A# - 2
B - 3
C(right) - 4

One end of the buttons (keys) are connected but other ends are still open. The other ends of the keys are the stoppers and the aluminium strip glued on the base.

Connect wires coming out of the stoppers and strip to the ground (GND).

Now the wiring is also done. We can now program our arduino in next step.

Step 8: Programming

As we know that this piano has two parts. One part is the physical cardboard piano which we made and another one is the computer in which our processing code will be running and tones will be generated. So we have two pieces of code, one for arduino and another one for processing. I am providing both the codes.

Connect the arduino board to the computer.
Upload arduino code into arduino board.
Run the processing code in the computer using processing IDE.

Press each key individually to check whether the keys are working or not.

Note: There has to be a proper contact between both part of the buttons to work i.e. between black keys and the stoppers and between white keys and the aluminium strip pasted on to the base.

Step 9: Final Assembly

After checking everything is working properly we need to enclose our piano. Use the schematics provided and cut the pieces of cardboard and paste it onto the base of the piano. Do not paste the upper part of the enclosure unless and until everything is working properly and well placed.

Color the keys i.e. black for black keys and white for white keys.
You can also color the enclosure of the piano using your creativity, but I prefer it to have plain cardboard look as the name says cardboard piano.

That's it, if you made it then you are awesome and you added one more cool project in your list. If you haven't made it yet then do it now, hurry. And as always play your cardboard piano and impress your girlfriend (if you have one :p).