How a rising edge detector turns a press into one pulse

A clocked circuit sees a held button as a 1 on every tick, so anything it triggers happens over and over. An edge detector turns that long 1 into a single one-tick pulse at the moment of the press. It takes two D flip-flops and an AND gate: one flip-flop holds the button's state now, the other its state a tick ago, and a press is the one tick where now is 1 and before was 0.

Held is not pressed

Hold a button for a second in front of a 10 Hz clock and that's ten ticks of "pressed". If each tick toggles a lamp or adds one to a count, one press does it ten times. What you want is something that happens once per press, however long the finger stays down.

Now and a tick ago

FF1 samples the button on each clock edge. FF2 copies FF1's output on the same edge, so it always holds what FF1 held one tick earlier. Then:

PULSE lights for exactly one tick per press, whether the press lasts a tick or a minute: a rising-edge detector. Swap the inputs, FF1's Q̄ AND FF2's Q, and it marks the release instead: a falling-edge detector. An XOR of the two samples marks both.

Push on, push off

Feed PULSE into a third flip-flop through an XOR with its own output:

On the pulse tick, Q XOR 1 flips the lamp. On every other tick, Q XOR 0 keeps it. One press turns it on, the next turns it off, and holding the button does nothing more. The lamp changes up to two ticks after the press, 0.2 s at 10 Hz, quicker than anyone notices.

Why not clock from the button?

You could wire a debounced button straight to a flip-flop's clock pin. But in an FPGA or a large chip, every flip-flop shares a few dedicated clock wires that reach the whole chip at the same instant. A button on a clock pin starts a second clock, and signals passing between the two can be caught mid-change. Detecting the edge and using the pulse as an enable keeps everything on one clock. That's synchronous design, the first rule of FPGA work, and edge detectors sit on nearly every input an FPGA reads.

The same idea in code is two variables: if the button reads pressed now and didn't last time, act, then save this reading for next time. Every power button, torch mode button and play-pause control works this way.