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Sidestepping the Arduino Header Offset with JeeLabs' Interposer Board

Gareth Halfacree

JeeLabs' Arduino header interposer board

The Arduino platform is famous for many reasons, from its open source nature to its approachability and flexibility. It's also famous for another, less appealing, reason: an early design flaw which is often the cause of wailing and gnashing of teeth. For anyone who has worked with integrating Arduino hardware into PCB-based projects, two simple words are enough to trigger this reaction: offset header.

It's immediately visible on inspection of the Arduino Uno, or any Arduino based on the same layout: while the majority of the headers are aligned to a 2.54mm (0.1") spacing, one header is offset on a 1.524mm (0.06") spacing. The result: a unique layout that makes it far from simple to attach the Arduino to boards designed around the ubiquitous 2.54mm spacing, and vice-versa, including solderless breadboards. While this design quirk has its arguable advantages - particularly in the design of Arduino-compatible Shield add-ons, which become keyed for orientation and protected against accidental connection to a non-Arduino board which may result in incorrect operation or even damage - it has been the cause of more than a few headaches, too.

Jean-Claude Wippler, of JeeLabs, is one of many frustrated by the Arduino's offset header issue, and is publishing a series of blog posts this week describing his solution to the problem. The first describes the issue, while the second introduces the Bridge Fifty-Fifty: a simple interposer board, which he has promised will be released for public use, designed to sit between the oddly-spaced Arduino and a more usual 2.54mm board.

Jean-Claude has also demonstrated how the design can be adapted to work in the other direction: a change to the board's layout provides a means to connect Arduino Shields to the non-Arduino Hy-Tiny microcontroller board. Using Jean-Claude's design as a base, then, it's possible to side-step the classic Arduino design flaw in either direction - albeit at the small cost of having a few interposer boards printed.

As you can see, all headers are doubled-up on the inside, without that 0.06” alignment error of standard Arduino shields. This board can be used with run-off-the-mill prototype boards with a “sea of holes” on a 0.1” grid.

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