Spark Fun has a simple breakdown of the various ways to interface 3.3V logic devices to 5V logic devices –
If you’ve ever tried to hook up a 3.3V sensor to a 5V micro, you know what I’m talking about – connecting these two can be a problem! There are several ways in which a 3.3V device can be safely connected to a 5v microcontroller. This tutorial will detail a few ways in which this can be accomplished.
Below is an example of how NOT to connect a 3.3V device (like the SCP1000-D01 pressure sensor with SPI interface) to a 5V microcontroller (like a PIC or AVR running at 5V). Although you may experience normal operating conditions, the lifespan of your 3.3V device will be dramatically shortened.
Spark Fun Electronics – Link.