Whether granting access to public transit or restricting unauthorized personnel in buildings, NFC card readers can be extremely useful. Although most might not consider how they work – and simply happy getting through a turnstile – there’s a lot going on behind the scenes.

In his video, Daniel Raines shows off a pair of prototype access control units (ACUs) that he’s constructed. The two networked devices are each based on a Raspberry Pi Compute Module 4 along with an Arduino Micro that controls six relays to allow or deny entry, provide feedback, fire, and lock up.

More details on the project can be found in Raines’ clip below.