Build an Ultrasonic-based Access System
You can make access systems using all sorts of input devices, as fundamentally - they all require a sequence of numbers entered to match those preset by the person in control.
And as you know, almost any sensor can create a numerical output - and in this case, an ultrasonic distance sensor has the duty of being the input device. The access system shown in the video below requires one to hold several different positions in front of the sensor for access:
Quite a clever idea, and an interesting take on access systems. For complete details, please check out the tutorial page. And if you're interested in doing this yourself, PMD Way can help you with the parts, such as: