Converting Arduino Boards
When Arduino debuted, most sensors and other interfacing devices ran on 5 volts. This tutorial shows how to hack your 5 volt Arduino board to run on 3.3 volts. It is a simple procedure involving replacing the voltage regulator and fuse.
Convert an Arduino to 3.3V which for a long time was the standard voltage for hobbyist electronics and microcontrollers. But now the coolest new sensors, displays and chips are 3.3V and are not 5V compatible. If you tried to connect to them with 5V it could damage the internals of the accessory.
Use chips like the CD4050 instead to do level conversion but if you are using a lot of 3.3V devices, maybe it would be better to upgrade the entire Arduino to run from 3.3V. To do that, replace the regulator so that the DC barrel jacks for a 3.3v type, and then reconfigure the 5V usb power line so it goes through the regulator as well.