logits.nexul.in

No install, no account, nothing to save to a server.

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Logits is a browser app for designing and simulating digital logic on an infinite canvas. Drag a gate out of the palette, click one pin then another to wire it, flip a switch, and watch the signal actually propagate, in simulated nanoseconds, through an event-driven engine.

It is not a picture of a circuit. It is the circuit.

40 element types   ·   9 categories   ·   4-valued signals   ·   buses up to 64 bits

What you get

Everything is a node	Gates, switches, clocks, flip-flops, counters, muxes, ALUs, ROM/RAM, splitters, tunnels, scopes, 7-segment and hex displays. All 40 come from the same definition contract, and none of them is special-cased anywhere in the app.
Real timing	Gates have propagation delay. Clocks drive edges. An oscilloscope records transitions on a ring buffer and draws the waveform on the canvas, next to the circuit making it.
Four-valued logic	0, 1, X (unknown or conflict) and Z (floating). Tri-state buses, uninitialised flip-flops and driver conflicts are things you can see rather than things that silently become zero.
Wires that behave	Free-angle routing drawn click by click, bends you can grab and move, branches pulled off an existing wire. Each lands as one undo step.
Deterministic	No Math.random, no wall-clock reads in the engine. The same circuit and the same inputs produce the same waveform every single run.
Local-first	A circuit is a plain JSON document. It round-trips through localStorage, export and import. There is no backend and no account.
Help where you are	Every element carries its own Markdown docs, and the pin and setting tables in the help dialog are derived from the definition, so they cannot drift from what the node actually does.

Signals

A signal is never a boolean and never a number bitmask. X gets a marker, not just a colour. The same rule holds everywhere in the UI, so state is never communicated by colour alone.

Try it without building anything

logits.nexul.in ships seven worked circuits in the sidebar. They open fully editable and simulatable, but are never written to storage, so break one freely, or import it into a project of your own.

Example	What it shows
Gate sampler	Every basic gate driven by the same two switches
Half adder	Two bits in, sum and carry out
Full adder	Two half adders and an OR, carry chained
2→1 mux	Select picks which input reaches the output
SR latch	Cross-coupled NOR gates that remember a bit
Master-slave flip-flop	Two latches in series, clocked on opposite edges
Tri-state bus	Two drivers sharing one net, with Z and X

Getting around

Gesture	Action
Wheel / two-finger scroll	Pan · Ctrl/Cmd + wheel or pinch zooms at the pointer
Click a palette element, then the canvas	Place it. Click the entry again to arm more copies, and up to 6 land as one undo step
Ctrl/Cmd + K	Command menu; places at the last pointer position
Click a pin, then a second pin	Wire them. Clicks on empty canvas in between drop bends. Esc cancels
Right-click a wire and drag	Branch a new wire from that point
Drag a wire, or drag a bend handle	Add or move a bend. Drop a bend on its neighbour to straighten
Space (tap) / .	Play–pause / single step
R · Ctrl+D · Ctrl+Z	Rotate · duplicate · undo
Tab, then Enter on a pin	Wire from the keyboard, because nodes and pins are real focusable DOM
Ctrl/Cmd + B · Ctrl/Cmd + J	Toggle the projects and elements sidebars

The full list, including the touch gestures, is in artifacts/07-interaction-spec.md.

Running it locally

Requires Bun. Use bun, not npm or pnpm.

bun install
bun run dev      # http://localhost:3000

bun run test     # vitest, node environment
bun run lint     # biome check
bun run build    # run this before claiming a change compiles

How it is put together

Three layers, and the direction of the arrows is the whole design:

• src/lib/ is pure. No React, no next/*, no DOM, no window. It runs in a plain Node test, which is why the 30 test files can simulate a ring oscillator or an SR latch headlessly.
• src/state/ is the only place the two worlds meet. Document mutations go through commands, so undo and autosave have exactly one hook. The simulation never drives React state per event: it notifies once per frame, and components subscribe per net through useSyncExternalStore.
• src/components/ draws. Nodes are DOM (so they are focusable, labelled and reachable by keyboard), wires are one SVG layer, and there is exactly one pan/zoom implementation for everything to build on.

Read artifacts/02-architecture.md before moving a file across a boundary, and the ADRs for why any of it is the way it is.

Adding an element

A definition file under src/lib/nodes/, and one line in src/lib/nodes/registry.ts. That is the entire change:

export const probeNode = defineNode({
  type: "io.probe",          // part of the save format, stable forever
  title: "Probe",
  category: "io",
  docs: `A numeric readout of a whole bus, in the radix you choose. …`,
  defaultParams: { width: 1, radix: "binary" },
  view: "readout",           // a name, not a component: this layer can't import React
  paramsSchema: [
    { key: "width", label: "Bit width", kind: "int", min: 1, max: 64 },
    { key: "radix", label: "Radix", kind: "select", options: [/* … */] },
  ],
  pins: (params) => [
    { id: "in", name: "IN", direction: "in",
      width: intParam(params, "width", 1), side: "left", offset: 2 },
  ],
  size: () => ({ width: 6, height: 4 }),
});

The palette, the command menu, the inspector form, the help dialog, the netlist and the simulation all pick it up from there. If you find yourself editing the canvas, the engine or the inspector to make an element work, the abstraction is broken, so fix that instead. Start with artifacts/05-node-authoring-guide.md.

Deliberately out of scope

Analog and transistor-level simulation. HDL import/export. Real-time collaboration. Accounts, a backend, a database. Logits is a logic canvas, not a schematic-capture tool and not a teaching platform.

Documentation

Design docs live in artifacts/ and are the source of truth for intent; src/ is the source of truth for what exists. Start at artifacts/README.md; the roadmap carries the current phase and an honest list of known issues.

Built with Next.js 16, React 19, TypeScript strict, Tailwind v4 and Bun.

License

Logits is source-available, not open source. It is licensed under the PolyForm Noncommercial License 1.0.0.

In plain terms: use it, study it, fork it, change it and share your changes freely for any noncommercial purpose — personal projects, hobby work, teaching, coursework, research. Build whatever you like on top of it for yourself.

What you may not do is use Logits, or the parts that make it Logits, for a commercial purpose — selling it, hosting it as a paid service, or shipping it inside a product you charge for. That needs written permission; ask.

Contributions are welcome under the same terms: open an issue for a bug or a feature request, or send a pull request. CONTRIBUTING.md has how to get set up, the rules that are load-bearing, and how to add a node. By contributing you agree your contribution is licensed to the project under the licence above.

Security

Found a vulnerability? Please report it privately — see SECURITY.md.