Michael Klements’ laser-cut, Arduino-controlled “safe” may not be the most secure model that you can find, but it certainly makes for a very entertaining puzzle.
Users can place whatever “treasure” they want hidden inside, shut the door, and press a rotary encoder button to lock it via a micro servo. They then must decipher a randomly generated four-digit code to get it open again.
Number guesses are input using the encoder dial on the front, which are displayed by a small OLED I2C screen. Green and red LEDs provide feedback as to how many digits are correct and if they’re in the right position, eventually letting users figure out where everything goes by a process of elimination.