Light Up Techniques

Seven techniques solve every Light Up puzzle on PeerPuzzles. Satisfied numbers and numbers that need every square get you started; lit squares and one way to light do most of the rest; number corners, what-if and leaving a square dark take over on the bigger grids. Each page shows its technique on the same 12×12 puzzle from the game, at the moment it’s first needed, so you can follow one solve from page to page.

Hard level 4, the 12×12 every technique page follows. It takes 42 steps and needs all seven techniques.

How many of the game’s levels need each one, out of 50 in each track:

Technique	Easy 7×7	Medium 10×10	Hard 12×12
Lit squares	50	50	50
A satisfied number	46	47	50
A number that needs every square	50	50	50
One way to light	48	49	50
Number corners	19	27	49
What if	0	0	50
Leaving a square dark	0	13	27

“Needs” means the hints, which always use the simplest technique that works, used it at least once. For all seven in one place, with another worked puzzle, see how to solve Light Up.

Hard level 4, solved.

Guides

Light Up: Lit SquaresThe most used Light Up technique: no bulb can stand in another bulb's light, so every square a bulb lights gets a dot. With a real example.
Light Up: A Satisfied NumberWhen a Light Up number already has all its bulbs, the other squares beside it get none. A 0 has all its bulbs from the start. With a real example.
Light Up: A Number That Needs Every SquareWhen a Light Up number has just enough room left for the bulbs it needs, every one of those squares gets a bulb. 4s, 3s on edges and 2s in corners. With a real example.
Light Up: One Way to LightWhen a dark Light Up square has only one place left that could light it, the bulb goes there, sometimes on the square itself. How to spot it, with a real example.
Light Up: Number CornersA bulb diagonal to a Light Up number lights two of its squares at once. When that would leave the number short of room, the diagonal gets no bulb. With a real example.
Light Up: What If? Follow a Choice to a ContradictionWhen no simpler Light Up technique works, suppose a square has a bulb (or doesn't), follow what it forces, and see whether a rule breaks. With a real example.
Light Up: Leaving a Square DarkA bulb that would rule out every way left to light another Light Up square can't go there. The rule, and a real example.