A quick demonstration using four JK flip-flops set up in "toggle" mode to implement an asynchronous 4-bit binary counter. This is the classic circuit, straight out of a text book. It's implemented with:

• two 74LS73 for the flip-flops
• a 555 Timer astable oscillator providing the clock pulse

As always, all notes, schematics and code are in the Little Arduino Projects repo on GitHub.

Why "asynchronous"? This refers to the fact that the output of each flip-flip cascades to trigger the next in series. Although instantaneous to the eye, there is none-the-less a propagation delay as the changes ripple through the flip flops. It's clearly seen in an analog scope trace. Here is the falling edge of 0xF transitioning to 0x0, which ripples over about a 42ns period:

Here's a quick video of the circuit in action ... though definitely not the most exciting thing you'll see on YouTube today: