#346 NAND Gate SR Latch
Set-Reset latch implemented with NAND gates.
Here’s a quick demo of it in action..
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:
- LEAP#061 SRLatch - using BJTs
- LEAP#077 SRLatchWithNorGates - using NOR gates
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 |
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:
Credits and References
- 74LS00 datasheet
- NAND-gate Latch
- SR Flip-Flop: NOR or NAND? - stackexchange
- LEAP#061 SRLatch - implementation using BJTs
- LEAP#077 SRLatchWithNorGates - implementation with NOR gates
- ..as mentioned on my blog