The EZ-Expander shield is an easy and inexpensive way to add digital output pins to your Arduino. This is accomplished by utilizing two 74HC595 shift registers. There are 16 new output pins on the shield (numbered 20-35), and the shield itself uses 3 existing Arduino pins to operate (pins 8, 12, and 13), so overall you get 13 additional output pins to use. I’ve provided an open source software library that makes it extremely easy to access the new pins 20-35 without having to worry about controlling the shift registers in your Arduino sketch.
EZ-Expander shield for Arduino