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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..



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



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:




Credits and References

About LEAP#346 Logic GatesCMOS/TTL
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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!).

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