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.

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.

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.