Expand your project possibilities, with the Adafruit PCF8575 GPIO Expander Breakout - an affordable 16 channel I2C expander.

GPIO expanders work like this: you have a board with some number of GPIO but not enough for your project - maybe you need more buttons or LEDs. You could upgrade to a board with massive number of GPIO like the Grand Central, or you could pop on one of these boards. Connect it over I2C and then you can send/receive I2C commands to control the GPIO pins to write and read them. It's going to be slower than direct GPIO access, but maybe that doesn't matter if it takes a millisecond instead of a microsecond. You only need the two I2C pins, and you can even share the I2C port with other sensors and devices. Heck, you can even add more expanders for massive I/O control!

The PCF8575 is a common and slightly unusual I2C expander for folks who are used to the MCP230xx series:

• First up, it's very affordable - who doesn't love that?
• It has 16 I/O pins
• Three I2C address select jumpers mean up to 8 expanders to one bus for 128 total GPIO added
• Each pin can be an input with light pull-up or an output sink
• IRQ output will automatically alert you when input pins change value
• This chip does not have a pin direction register. You cannot set the pins as input or output - instead, each pin has two possible states. Basically, you can think of it as an open-drain output with a 100K resistor pull-up built in.
• Option one: Lightly pulled up 'input' - by default it will read as a high logic level, but connecting the GPIO to ground will cause it to read as a low logic level.
• Option two: Strong 20mA low-driving transistor sink output. This means the output is 'forced' to be low and will always read as a low logic level.