Sandy Macdonald has taken a Pimoroni Explorer HAT and used it to create a PIN entry system. At the moment, the entry system lights up some LEDs and makes a buzzer sound, but obviously it could be extended to unlock a door, a box, or whatever you choose. He’s written it up as part of the Pimoroni learning portal but has done a brief write-up of it on his blog and linked from there to the Github repository containing the full write-up. Read the brief summary here or visit the full write-up here.