The humble H-bridge is the most common means of controlling DC motors from an Arduino or other microcontroller. Allowing for bidirectional control - unlike a simple relay or transistor, which could only run the motor in a single direction - the H-bridge is handy, but its typical packaging as a black integrated circuit makes it hard to guess what's actually going on under the hood.
Curiosity as to how an H-bridge actually works drove maker Mayukh Nath to to investigate its inner workings. Discovering that the circuit hidden beneath the plastic casing is surprisingly simple, Mayukh designed his own as a means of easily demonstrating what goes on inside the IC when you're controlling it via the Arduino.
While Mayukh's homebrew H-bridge isn't suitable for driving motors, it lights up LEDs depending on which pair of switches are depressed - indicating which motor output would be active when the inputs of the H-bridge are signalled by the Arduino. Great for classroom use, the project requires only a handful of parts: four pushbutton switches, two LEDs, a power source, and a breadboard with jumper cables for the wiring.
As a high school student who loves electronics with a free summer, I started playing with Arduinos and one of my first questions was "how can I spin a motor backwards?" A quick Google search led me to the H-bridge, one of the simplest and most common ways to obtain bidirectional control of a motor. The first time I saw this, I assumed it was one of those complicated things I wouldn't worry about but when I saw the diagram I was surprised by its simplicity and ingenuity and couldn't help but make one myself.