One the of very first projects I did with Printed Circuit Boards (PCBs). I bought all the materials from Adafruit, a great website with circuit board project instructions you can download.
The code you’ve saved to the circuit board listens to a little speaker for a sequence of knocks (taps) you pre-recorded. If you knock/tap the correct code, the circuit board unlock a door latch (a solenoid-type door latch that is electronically controlled).
Photo credit and for instructions for this build, visit this Adafruit page.