Signal Flip

Eight switches, labelled A to H. Pressing one flips itself and a published set of the others — C flips C, E and F, and the list is printed under the board rather than hidden. Some switches start on, some start off, and your job is to get all eight on inside the press budget.

The catch, and the whole point: pressing a switch twice is the same as never pressing it, so the order you press things in makes no difference at all. What you are looking for is not a sequence but a set. Every board here is wired so that exactly one set works, and that is checked before the board is dealt rather than hoped for afterwards.

It runs entirely on your device: 0 network requests, works offline. No ads, no account, nothing to buy, and no betting, money or prizes of any kind.

1. Focus the board — click it, or press Tab until it is focused.
2. Move along the row with the left and right arrows. Up and down walk the same row in reading order, so on a narrow screen where the switches wrap onto two lines no arrow key is wasted.
3. Read what the cursor would do. Every switch the one under the cursor would flip gets a dashed edge, so you can see the reach of a press before you spend one.
4. Press Enter to flip the switch under the cursor. That costs one press from the budget.
5. Read the wiring list under the board. Each row gives both directions: what pressing that switch flips, and which switches it is flipped by. The second one is what the deduction runs on — a switch that only one thing reaches tells you immediately whether that thing is in the answer.
6. Watch the parity. Each switch says even or odd underneath: how many times you have pressed it. A switch reading even is doing nothing at all right now, because its two presses cancelled.
7. Click a wiring row to press that switch as well. Same press, bigger target — useful on a phone.

How do you solve a switch puzzle where pressing one switch flips several others?

Stop thinking about order and start counting how many times each switch gets flipped. Every press is its own undo, so all that matters for each switch is whether the presses touching it add up to an even or an odd number. That turns the whole puzzle into eight small true-or-false questions — "is A in my answer set?" — instead of a search through press sequences.

Then find the easiest question and answer it first. Look down the flipped by column for a switch that only one or two things reach. If a switch is off and only A reaches it, then A must be in the answer, because nothing else can ever turn it on. If a switch is on and only A reaches it, A must be out, because pressing A would break it. Pin that switch, cross it off, and every remaining question gets shorter. Working from the most constrained switch outwards solves these boards without a single guess — which is exactly how you solve any system of equations by hand.

Is there always exactly one answer?

Yes, and both halves of that are proven before you see the board. The answer set is drawn first and the board is built by applying it to the finished, all-on state — so pressing that same set again is guaranteed to clear it, and no board can be impossible. Then the wiring itself is checked for invertibility by Gaussian elimination over the two-element field, and a wiring that fails is thrown away and redrawn. An invertible wiring is precisely one where every board has exactly one solution set, so there is never a second answer hiding, and never a board with none.

To make doubly sure, the game also walks all 256 possible press-sets and counts how many clear the board. Two independent arguments agreeing beats one argument trusted.

Why does the order I press them in not matter?

Because flipping is addition where 1 + 1 = 0. Pressing a switch adds its wiring to the board; adding the same thing twice cancels out, and additions can be done in any order. Mathematicians call the result a linear system over GF(2), the field with two elements, and it is the same object that sits underneath error-correcting codes and parity checks in real hardware. It is also why the game shows you an even/odd count rather than a press log: the log would be noise, the parity is the state.

What happens when the presses run out?

The round ends and the answer is shown, along with the set that would have finished the board from wherever you stopped. That is the only way to lose. The budget is the shortest possible solution plus three spare presses — the shortest is computed exhaustively for your board, not guessed — so it is always enough to solve it and never enough to flip switches at random until something works. Moving the cursor and reading the wiring are free and unlimited.

Is there any money, betting or in-app purchase in this?

None, and there never will be. No currency, nothing to buy, no hints to unlock for a fee, no prizes.

Does it work offline?

Yes. Once the page has loaded once, the whole game — the wiring generator, the rank check that proves the answer is unique, and the exhaustive solver behind the budget — is on your device. No server is involved in play at any point.

Can I play it with a keyboard or a screen reader?

Both. Arrows move along the row and Enter presses. Every switch announces its letter, whether it is on or off, how many times you have pressed it and whether that is even or odd; every wiring row announces what it flips and what flips it. On screen, on and off are a filled and a hollow disc plus the words "on" and "off", and a switch within reach of the cursor carries a dashed edge — so nothing on the board depends on telling two colours apart.

Eight switches, so 256 possible press-sets — small enough that the minimum is found by exhaustive search rather than estimated, and small enough that uniqueness is counted rather than argued. Each switch flips itself plus one to three others, drawn fresh for every board, and the answer is always three to five switches. The budget is that shortest answer plus three spare presses, so boards run from six to eight presses. The wiring is redrawn until it is invertible, up to forty attempts, after which a known-invertible fallback wiring is used — so generation always terminates.

Light Grid is the same mathematics with a fixed neighbourhood: press a cell and its four neighbours flip. Gap Logic is the other deduction puzzle here, and turns on order rather than parity.

Bookmark this page (Ctrl+D, or ⌘D on Mac) or install the app — it works offline the next time you need it.