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: