Light Up Puzzle Generator
Light Up asks you to place bulbs on a grid so that every white square is lit while no bulb shines on another. A bulb lights its whole row and column until a black square blocks it, which makes every placement a long-range decision: nothing can be settled by looking at one square. The numbers on black squares say exactly how many bulbs sit beside them, and between them they pin the answer down to one.
What this generator does
Lays out black squares, finds a legal bulb placement, numbers the black squares from it, then rubs out every number that turns out not to be needed — checking after each removal that only one answer remains.
How to use this tool
- Pick a grid size and generate a puzzle.
- Place a bulb in a white square; it lights the row and column until a black square stops it.
- A numbered black square must have exactly that many bulbs directly beside it.
- Finish when every white square is lit and no bulb can see another.
Understanding the controls
- Grid size
- From six squares a side to nine. Larger grids hold more bulbs and take longer to settle on a single answer, which is why nine is the ceiling offered.
- Seed
- A word reproduces exactly the same grid, so a puzzle can be shared or set for a group by naming the seed rather than copying the picture.
Common use cases
- A fresh logic puzzle to print or solve on screen
- Puzzle pages for a newsletter or club sheet
- Practice at constraint reasoning that is not arithmetic
- Demonstrating a puzzle with exactly one answer
- A short, self-contained break that still takes thought
How this generator works
The published grid is solved again from its numbers alone, counting answers rather than assuming the construction was sound. A grid is only shown when that count is exactly one and the answer found matches the one held. The pared-back numbering is minimal in the strict sense: rubbing out any one of the numbers that survived would leave a puzzle with more than one answer.
Randomness and fairness
The black squares and the bulb placement come from a random draw. Give a seed and the same word always reproduces the same grid — that path is deterministic, not cryptographic, and is meant for sharing a puzzle rather than for anything that needs to be unpredictable. Leave the seed blank and the browser's secure generator is used instead.
For how randomness is produced across the whole site, see how Generate Random works.
Limitations and good to know
- Grids stop at nine squares a side. Beyond that, confirming a single answer takes long enough to be felt.
- Difficulty is not graded. A minimal set of numbers can still leave an easy puzzle or a hard one.
- No solving aid or hint is offered — the answer is shown whole or not at all.
- Nothing is stored between visits; leave the page and the puzzle is gone unless you noted the seed.
- The grid is drawn for the screen rather than laid out for print.
Privacy and your data
Grids are built in your browser. No puzzle, answer or seed is transmitted or stored.
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