Instead, it relies on an ultrasonic distance sensor to detect a hand (or foot) waved in front of the door. An Arduino Leonardo runs the show, sounding a buzzer when the ultrasonic sensor is triggered.

Ultrasonic sensors work using ultrasound to build up an image of your fingerprint (yes, really) and work better with messy fingerprints - if your hands are wet or oily with sun cream, for example.

Redmi, a Xiaomi spin-off phone brand, says it has made in-display fingerprint sensors that work with LCD screens. Previously, only OLED screens supported them.