Expansion boards like the Arduino 4 Relays Shield (Fig. 2) is why these platforms have become so popular. It’s allowed third parties to give developers access to peripherals from wireless ...

[Allan Schwartz] decided to document his experience using Fritzing to design, fabricate, and test a custom Arduino shield PCB, and his step-by-step documentation makes the workflow very clear. Anyo… ...

The ‘under’ in Undershield comes from this shield being mounted underneath the Arduino, unlike every other shield on the planet.