An ultrasonic sensor reads the output value and sends the result back to a PC. One interesting problem that [Pepijn] ran into, and which he explains in great detail on his blog, is that LEGO gears ...

and is fitted with a Lego “fretboard” of sorts. A slide is moved up and down the fretboard by the player. The EV3 controller detects the position of the slide via an ultrasonic sensor ...