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Project Notes

#348 D Latch

Building a D Latch with NAND gates (74LS00).

Here’s a quick demo of it in action..

Build

Notes

The Gated D latch:

  • sets the complementary outputs (Q & ~Q) based on a single input bit (“D” i.e. data)
  • out changes are synchronous i.e. no clocking required - they change immediately
  • new data is only latched when the enable input is set
  • if enable input is not set, the outputs (Q & ~Q) remain latched at their last value

The gated D latch is a fundamental 1-bit memory unit, and is at the core of much digital logic design.

Gated_D_latch_symbol

Truth Table

E D Q(t+1) ~Q(t+1)
0 0 Q(t) ~Q(t)
0 1 Q(t) ~Q(t)
1 0 0 1
1 1 1 0

Logic Design

There are various combinations of gates that can be used to construct a D latch.

I’m going to test a design based on an SR NAND latch, which means a single 74LS00 (quad two-input NAND):

D-Type_Transparent_Latch

Alternatively this can be built based on an SR NOR latch:

D-type_Transparent_Latch_%28NOR%29

Ben Eater’s Tutorials

Great explanations of the D latch and D Flip-Flop..

D latch

D flip-flop

Construction

I’m building this first on a breadboard using a single 74LS00 (quad two-input NAND), LED indicators and manual inputs.

  • the enabled input uses one pole of a DPDT switch
  • the data input uses a push-button with the default position low with a 1kΩ pull-down resistor

NB: the pull-down resistor can’t be too stiff with the 74LS00. For example, a 10kΩ pull-down resistor allows the input to float a little too high and corrupt the input.

Breadboard

Schematic

Build

Credits and References

About LEAP#348 Logic GatesCMOS/TTL
Project Source on GitHub Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is my personal collection of electronics projects, usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting (IMHO!).

The projects are usually inspired by things found wild on the net, or ideas from the sources such as:

Feel free to borrow liberally, and if you spot any issues do let me know. See the individual projects for credits where due. There are even now a few projects contributed by others - send your own over in a pull request if you would also like to add to this collection.