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Project Notes

#346 DigitalLogic/SRLatchWithNandGates

Set-Reset latch implemented with NAND gates.

Here’s a quick demo of it in action..

Build

Notes

This circuit is another implementation of a Set-Reset flip-flip, this time using NAND gates. A 74LS00 is used to provide the two NAND gates required.

Other implementations:

The NAND gate implementatation has an undefined state with both inputs high, whereas a NOR gate implementatation has an undefined state with both inputs low.

Consequently NOR gates are typically used for active-high SR latches and NAND gates for active-low SR latches. A good example of this is the application in touch sensors, where active low makes sense. See the Boldport Club tap and mostap projects for examples of this.

Truth Table

Set(t) Reset(t) Output Q(t+1)
0 0 invalid state
0 1 1
1 0 0
1 1 Q(t) no change

sr-latch-nand

Construction

Here’s a quick breadboard build using a 74LS00 quad two-input NAND gate, with some switches and LEDs to indicate S, R, Q and `Q:

Breadboard

Schematic

Build

Credits and References

About LEAP#346 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.