This is a simple cardboard project that you can recreate with your child and as the name say's it's not a Piggy bank but a surprise cardboard box.when you put a coin inside,the box will pop open in seconds revealing what's inside.you can writet a message or put some Candy's inside it.

Supplies

Twelve Cardboard squares (8cm×8cm)
Paper Glu
Paper
Rubber band
Additional supplies(not important)
LED
Battery
Wire
Metal piece
Ribbon cable
Stickers

Step 1: Cutting Pieces

To make this box we need twelve pieces of 8cm ×8cm cardboard squares. On one piece cut a small opening for insert ing the coin. in this twelve pieces six will be used to make the box and the remaining six to hide all rubber band Glu marks and paper
Next we need to cut a triggering mechanism which will open when a coin is inserted in the box. Shape and size of these parts are shown above (second picture)but you may need to make some change since cardboard thickness can also affect the mechanism.so don't just follow these instructions only make your own changes if needed.

Step 2: Gluing All Pieces

Arange all 8×8 pieces of cardboard as shown in the first picture then connect each of them to the next one using paper and paper glue but do this only on one sideso it can only fold in one direction.these papers will hold the cardboard in place under tension this is important because if the pieces shifts or slides there won't be enough tension on the rubber band.After gluing the papers flip it upside down on a flat surface and Glu rubber bands .we can't use normal paper Glu for this because under tension it won't be able to hold the rubber band,so use super glue instead.use atleast two rubber bands on one joint and not more than four.its better to use elastic bands since it's easy to Glu on but if you don't have it its ok to use rubber bands as I did.after finishing you can see how this works you can only fold it in one direction and as soon as you take your hands off it will flip back into its original flat shape.

Step 3: Trigger Mechanism

Next we need to make a triggering mechanism.what I made initially was not perfect so the above drawing is a new better one.after cutting take the long piece(first one) and Glu it onthe first cardboard square it's bottom side is 3cm below the top edge of square and 3cm away form side edges.But when gluing Glu only bottom side with paper and Glu as we did earlier.so this piece will be able to move up and down easily.then glue a small thread in this piece and put it out through the small hole made above this trigger.so when pull on the thread the trigger will come up to its position.next we need to glue another tiny piece on this also but before that glue the second piece on the last squre.Glu this 2.5 cm below and 3cm away from edges so that when we Glu the final piece on the long trigger it will get locked inside this piece.now close all the side except the top side and Mark the position where the small piece is to be glued in the long trigger piece and glue it on using super glu. hope you understand what I explained above take a look at the pictures before doing this.

Step 4: Automatic Switch and Decorations

Here I'm using a red and blue blinking led.its a small single led chip with a built-in blinking circuit so there is no need of any other electronic components. but i don't know whether you can buy this or not i got it from an old toy.you can use normal led with its blinking circuit if you want. but the important part is the switch which turn on when the box is opened and off when closed
To make this we need a thin small metal piece a ribbon cable(normal insulated wire also works)and some wire.glu one side of the ribbon cable at the edge of middle cardboard piece other side is free to move.now in open position of the box glu the metal piece right below the wire so in this position the wire is in contact with the metal piece if its not making contact use a paper and apply Glu on both sides only and stick it over the ribbon cable.this way you can keep the ribbon cable flat and in contact with metal.when the box is in open position switch is on and when the box is closed wire slides away from the metal piece turning the switch off.next I soldered two batteries together and positive terminal is connected to the metal piece using insulated copper wire and negative terminal to one wire of the wire in the ribbon cable now this negative wire should not tuch the metal piece or the positive cable you can use insulation tape for this.now our switch is ready.from the other side of the ribbon cable solder positive and negative to the led chip.i gluede a small glass flower over this led chip and used the wire's as stem of the plant of this way you can avoid hiding the wire's.

Step 5: Clearing the Mess and Decorating

So now it's time to hide battery and all glu stains.use paper for this cut suitably sized paper and Glu it inside the box and over this paper you can write your message or add stickers.battery is placed in a small compartment cut in the box.For outside of the just stickingpaper won't work because of the rubber bands.so cut another six pieces of 8×8 cardboard and Glu this using double sided tape and over this you can Glu the paper.and for the edges glue paper piece's in closed position and fold it in to the junction.do this on all connecting joints.you can draw a face on top of the box if you want.to close the box first fold the bottom piece then sides after closing sides pull on The string to activate The trigger.then close the top side if the top side is not closing glue a small piece of cardboard in the top side so it will tuch the side and stay on place

Step 6: Finished

After closing The box put a coin inside it the coin will fall on the long trigger piece and the weight of the coin will cause the trigger to unlock and the box will pop open at the same time the LED will get turned on.also revealing what's inside.
If you like it try making one of your own and stay safe