Gate — Build the Circuit That Matches the Truth Table
Three inputs, three empty slots, one truth table to match. Pick the gates that build it. Six puzzles, free, no sign-up.
Six puzzles. Pick the three gates that make the circuit match the table.
How to play
- Match the target column. Tap a slot to cycle it through the five gates.
- Wiring: g1 takes A and B, g2 takes g1's output and C, g3 takes A and g2's output.
- No NOT gate — feed a wire into both inputs of a NAND or NOR to invert it. That is what makes those two universal.
- Four tests per puzzle. Each one only tells you which rows already match, so work it out rather than cycling.
How it works
Each puzzle gives you a truth table and a circuit with the wiring already done and the gates missing. You choose one of five two-input gates for each of the three slots so the circuit produces the target output for all eight input combinations. There is no NOT gate, and that is the lesson rather than an omission: to invert a wire you feed it into both inputs of a NAND or a NOR, which is exactly the fact that makes those two gates universal. Every puzzle is provably solvable — the set is derived by enumerating all 125 gate assignments — and the difficulty is the measured number of solutions, from four ways in down to exactly one. You get four tests per puzzle, and each test tells you only which rows already match, so searching is hopeless and reasoning is not. Runs on your device, nothing is sent anywhere, and there is nothing to sign up for.
A puzzle about boolean logic, not a circuit simulator. Real hardware has propagation delay, fan-out limits and a NOT gate.
Frequently asked questions
Why is there no NOT gate?
Because you can build one. Feed the same wire into both inputs of a NAND and you get its inverse — NAND(x, x) = NOT x — and the same trick works with NOR. That is what people mean when they call those gates universal: every other gate can be built out of them. Leaving NOT out of the palette is what makes you find that out.
Can every puzzle actually be solved?
Yes, provably. There are five gates and three slots, so 125 possible circuits, and the game enumerates all of them before it starts. A truth table is only offered as a puzzle if at least one assignment produces it, and the number that do is exactly how the difficulty is graded: some targets have four ways in, thirty-three have exactly one.
Can I not just try every combination?
Not in four tests. There are 125 assignments and at most four of them are right, so guessing scores about 20 out of a possible 600 — measured, not assumed. Cycling one slot and testing each time does slightly worse. The feedback after each test is deliberately thin: which rows match, and nothing about which gate is wrong.
How should I approach a puzzle?
Read the target column against the inputs and name the function first — is it "A and something", is it an exclusive or, is the output mostly true? Then work backwards through the wiring: the last gate sees A and one wire, so whatever that wire has to be is your real problem, and it is a smaller one.