We all have a smartphone. Now, with that statement of the obvious, let me ask you this. Why is your Arduino hardly ever connected to your smartphone. Bluetooth costs about $8 to implement on an arduino. The main issue is that some people get hung-up on the issue of programming bluetooth, so they just keep the usb cord. So here is the absolutely most basic things you need to use bluetooth with android and arduino.
This tutorial will go through the bare minimum you need to create an connection between a custom android app you make with MIT App Inventor and the Arduino.
All code and references provided are based on the code from the LittleArm Arduino robot arm.
The most common bluetooth module today is the HC-06 or HC-05. It costs about 7-10 dollars online. Look at the image below to see how to hook it to the Arduino.
That is pretty easy right? Just four jumper wires and you now have wireless serial connectivity to your arduino. Now, if you have a program where you have been sending commands to the arduino with the Serial Monitor you can now do that wirelessly with a bluetooth device. (Note: You can upload programs to the arduino via bluetooth, but that is significantly more complicated than what we want to achieve here.)