Anything can be used as a key as far as you know the password ;)
A fun way to make a lock. You set a password initially and the ultrasonic sensor every 3 seconds measure a distance(between the sensor and an object/card) and it compares the password set with the one you enter. If the password is valid, the lock will remain unlocked as far as the distance measured is greater than 5 cm hence, you can lock the box whenever you want.”