The MCP23017 HAT from ModMyPi is the perfect I/O expansion HAT for your Raspberry Pi A+/B+/2/3 etc. The HAT features two removable MCP23017 DIP IC's, which utilise two i2c pins (which can be shared with other devices if required), and in exchange gives your Raspberry Pi an additional 32 GPIO ports! Each I/O pin can be configured as an in input, output or an input with a pullup resistor. The MCP23017 HAT is stackable, featuring selectable i2c address jumpers which supports up to 8 independent addresses: This means you can add up to 128 additional I/O's to a single Raspberry Pi by stacking up to 4 MCP23017 HAT's!
The MCP23017 HAT can also be used to protect your Raspberry Pi! We've used removable, rather than surface mount, MCP23017 DIP IC's which can act as replaceable buffer for the Raspberry Pi's GPIO, essentially protecting your Raspberry Pi from damage via the GPIO ports. The MCP23017 HAT is powered via the Raspberry Pi's GPIO port, but features an included (but unsoldered) terminal block for advanced users wanting to use an external 5V source, or stack multiple MCP23017 HATS on a single Pi. The isolation jumper "PWR JUMPER" must be removed when using an external source.