Arduino Ultrasonic Theremin (HC-SR04 + NeoPixel Ring + Buzzer)

A musical instrument you play without touching it, just by moving your hand in the air! As your hand gets closer to the **HC-SR04**, the **passive buzzer** plays higher notes and the **24-LED NeoPixel ring** shows each note with color and fill level. A button switches between two modes: in **SCALE** mode every distance snaps to a note of a pentatonic scale, and in **GLIDE** mode the sound slides smoothly like an old sci-fi movie. Your hand writes the melody, not the code.