LearnTronics
The SR Latch — Electronic Set and Reset
The electronic version of Set and Reset
The relay seal-in circuit gave us a physical memory. An SR latch gives us a very similar idea using logic gates.
S means Set; R means Reset
For the active-HIGH NOR version shown here:
| S | R | Q(next) | Meaning |
|---|---|---|---|
| 0 | 0 | Q(previous) | Hold / remember |
| 1 | 0 | 1 | Set |
| 0 | 1 | 0 | Reset |
| 1 | 1 | invalid | Avoid for the basic NOR latch |
Compare it with START and STOP
The relay's START button resembles a Set command. STOP resembles a Reset command. Both arrangements can remember a state after the momentary command is gone.
They are not identical circuits, but the comparison helps connect mechanical control logic to electronic memory.
Why feedback creates memory
Each NOR gate receives the other's output as one of its inputs. Once one side wins, that output helps keep the opposite side in the complementary state.
That is why a simple truth table of only S and R is not quite enough; the previous Q matters during the hold condition.
Important notation: Q is the stored output. ~Q is normally its complement.