"You got the Ancient Circuit! It is made of shining gold. It also looks like it may be a part from...something. The surface is inlaid with circuitry."
— In-game description

The Ancient Circuit is an item from The Legend of Zelda: Skyward Sword. It is used as a key to enter the boss room of the Lanayru Mining Facility, where Moldarach is found. In order to open the door leading to the dungeon's boss, Link must correctly maneuver the circuit so that it fits into the lock.

See also