Stackable IO for the Raspberry Pi

Alasdair Allan
2 min readSep 21, 2017


One thing that surprised the Raspberry Pi Foundation, after they’d got over being surprised about how popular it was in the first place, was how much use was made of the GPIO. The Raspberry Pi, which had been targeted directly at the education market, proved to be far more popular amongst makers than amongst almost anyone else.

The Raspberry Pi has gone one to become one of the pillars of the maker movement. But there’s a problem, the GPIO is limited. Enter the Mega-IO HAT, now funding on Kickstarter.

The Raspberry Pi Mega-IO HAT. (📷: Sequent Microsystems)

Of the 40 pins in the Raspberry Pi header block, only 26 are digital GPIO pins and some have special purposes. There is only limited PWM capabilities, and no analog input and output pins at all.

The Mega-IO HAT passes through the Raspberry Pi’s own GPIO pins and adds an additional 8 on-board relays capable of switching up to 24V load, 8 12-bit ADC channels, 8 opt-isolated inputs, 4 open collector outputs, 6 additional GPIO pins, and a single 12-bit DAC output, all controlled from an 8-bit ST STM8L151C8 micro-controller.

Stack of three HATs with DIP switch to test opto-isolated inputs, and thermistor to test analog inputs. (📷: Sequent Microsystems)

All the with all the through-hole components are mounted on top of the HAT, with only surface mount components are on the bottom. The Mega-IO HAT is stackable up to four levels, with a two-position jumper allowing you to select the stack order of the card. The HAT should be controllable either via a command line tool, or from a Python library, or via Node RED. Both the command line utility and Node RED example source are available for download, along with the HAT’s user manual.

The Raspberry Pi Mega-IO HAT is fully funded on Kickstarter with just over a week to go on the campaign. Pledges start at $29 for a single expansion card with an additional $6 for shipping. Boards are expected to ship in April 2018.