You can build a drum kit pretty easily if you have Arduino, Bluetooth and a Smartphone. Here we have made a DIY Bluetooth drum kit that takes input from a Piezo and plays it from your phone.

The whole thing fits into a box and sound is played from your phone via Bluetooth. This make the everything portable

We did this as part of our kickstarter called Jetpack a wireless programing Bluetooth shield+motor driver shield for Arduino. You can use your own substitutes for this.

Got 750% funded on Kickstarter! We are amazed by this response and so jet pack is now available for pre-order on http://www.jetpackshield.com/

Things required

Arduino
Jetpack(Bluetooth Shield+wireless programmer)
Piezo(6)
100K Resistor(6)
Wires
Cardboard
USB Speakers With 3.5mm audio plug (this is optional,you can use your phone's speaker, just in case you like things really loud :) )

Step 1: Make Your Cardboard Drum Kit

I had a cardboard box lying around.. If you use a box you can hide all the wires and Arduino inside to carry it around easily.

Cut out round pieces from a cardboard box (this will be your drum pad). You can use anything you want, something easy to work on. If it has to last a few beatings you can make one from a mouse pad or something similar
Use spray paint or chart paper to colour your the box. Just make sure it looks cool :)
Now cut out small slots, where you want to fix your drum pads. We will route our wires from there.

Step 2: Connect Piezo

A Piezo sensor converts force into electric charge. We use the same principal here.

There are two points on a Piezo sensor. You need to solder a 100k resistor across the positive and ground(the silver outer ring) points on the Piezo.
Now solder two wires on both terminals of the Piezo(or resistor legs). The positive one will go to the jetpack's (same as Arduino's) analog pin and the ground(outer silver ring) to Jetpack's Ground. Have a look at the images given above for better understanding.
Complete the connections as shown above

Note if you are not using jetpack connect the Piezo directly to Arduino's analog pin and ground. You will need an additional Bluetooth Module to connect it with your smartphone

Step 3: Pack Everything and Upload Your Program Wirelessly With Jetpack

Power your Aduino with a 5v adapter(or battery),
Now if you have Jetpack you can program it over Bluetooth later. No worries, you wont have to unplug everything if you need to reprogram. You can do it OTW :) Or directly program it with arduino. Just make sure everything is working before you close it up.
I placed the Arduino after cutting a slot under the box, I have include the pictures. Also the code is given below.

*Refer this instructable (data monitoring) for wireless programing via jetpack. You will be using the outgoing Com Port when you upload the code