If you’re looking for a quick and dirty means of interfacing high and low voltage logic devices, consider the Texas Instruments CD40109. This CMOS device functions with supply and signal voltages of up to 20 volts, and can either shift voltages up or down depending on the ratio of Vcc (low supply) to Vdd (high supply).
NOTE: The datasheet has a typo at the end of the Description section. It should read, “When operated in the mode VCC > VDD, the CD40109 will operate as a high-to-low level-shifter.”
It sells for less than a dollar in the 16-pin DIP at Digikey.