D Flip-Flop LED State Indicator

This D flip-flop example makes the stored state easy to see. The data input supplies the value to be captured and the clock pulse determines when that value transfers to Q. An LED connected through a current-limiting resistor indicates the state held at Q after the clock edge. Q-bar remains available as the complementary output. The arrangement is useful for explaining why a flip-flop does not continuously pass its input to the output. It also gives a simple pattern for a status indicator in a small digital circuit, provided the output drive and LED current meet the chosen device limits.

Type Schéma électriqueNorme IEEE Std 315 / ANSI Y32.2Moteur schematex-circuitMis à jour 24/09/2026

La demande

“Draw a 5 V 74HC74 D flip-flop with a data input and clock pulse input, and connect Q through a 330 Ω resistor to a stored-state LED.”

Essayez ensuiteAdd an active-low clear input.Change the LED resistor to 1 kΩ.

Ce que contient ce dessin

Comprenez les décisions qui le sous-tendent.

01LED output

The LED makes the stored Q state visible.

02Series resistor

The resistor limits LED current.

03Clock input

A separate pulse source distinguishes capture from data level.

Use this as a classroom or breadboard demonstration of D flip-flop behavior.

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FAQ

Questions fréquentes

Why does the LED not follow D immediately?

Q changes when the flip-flop captures D on its active clock edge, not whenever D changes.

Why is a resistor used with the LED?

It limits current through the LED and protects both the LED and the logic output.

Can Q-bar drive another LED?

Yes, with its own current-limiting resistor and within the output drive limits.