Unit testing is finally making its way into embedded development, though most see testing as an impediment to completion of a task. Mocks provide a lightweight mechanism to simulate interactions with lower layers of the system. This empowers us to do simultaneous top-down design and development. We will take attendees through the full top-down implementation of a full project on an Arduino armed with only the C language, the GCC toolchain, and Ceedling, our bundle of free and open-source tools. Lucky attendees will receive an Arduino starter kit with all the goodies to replicate this project and take it home.