SR memory cell

Press one button to remember a 1, another to forget it. A ready-made memory circuit you can open in the TorchAnvil simulator.

A circuit that remembers

Every circuit you've seen so far is combinational — its output is a pure function of its inputs. Flip the lever, the lamp responds; let go, it goes back. There's no memory.

This one is different. Tap Set once: the lamp lights and stays lit — even after you release the button. Tap Reset: the lamp goes dark and stays dark. The circuit remembers which button you pressed last.

How it works

The SR flip-flop has two inputs (S for "set", R for "reset") and one state bit Q that survives between clicks:

• S high, R low → Q becomes 1 (lamp on).
• R high, S low → Q becomes 0 (lamp off).
• Both low → Q holds whatever it was. This is the memory part.
• Both high at once is ambiguous — the real-world latch picks one or gets confused. The simulator just holds the previous state, but try not to press both buttons at the same time.

S	R	Q (next)
0	0	Q (hold)
1	0	1 (set)
0	1	0 (reset)
1	1	unstable

Why it matters

This tiny block is how computers store a single bit. Wire eight of them up side by side and you've got a byte of memory. Add an address decoder and a clock, and you're most of the way to RAM.

Try this

• Tap Set then let go. Lamp stays on — no input is being held, yet the circuit is clearly "doing something."
• Tap Reset and let go. Lamp stays off.
• Tap Set a second time while the lamp is already lit. Nothing changes — it was already remembering 1.