Circuits and Combinators

The circuit network is Factorio’s wiring layer: it lets you connect buildings with red and green wires and control their behavior with signals. You can read the contents of a chest or tank, do arithmetic and logic on those signals, and feed the result back to switch a machine, a pump, or a train station on and off. None of it is required to launch a rocket, and many players finish the game without touching combinators. But circuits are what turn a factory that runs into a factory that regulates itself, and they unlock the compact, hands-off designs the late game rewards. Start small, with a single condition, and build up.

What the network carries

Buildings connected by wire share signals, and a signal is just an item or value with a number attached. A chest broadcasts how many of each item it holds; a tank broadcasts its fluid level. Other buildings can be set to act on a condition, so an inserter only swings when a signal crosses a threshold, or a pump only runs when a tank reads below full. The simplest useful circuit is exactly that: wire a storage tank to a pump and switch the pump off when the tank is full, so you never overflow or waste a fraction.

The combinators

Four combinators do the computing. Constant combinators emit a fixed signal you set by hand, useful as a target or a label. Arithmetic combinators compute on signals, adding, multiplying, or otherwise transforming them. Decider combinators apply a comparison and output a result when the condition holds. Selector combinators, added in 2.0, pick, sort, or sample signals, for jobs like grabbing the largest value on the wire. Chained together, these let you build counters, indexes, and thresholds out of nothing but signals.

The wiki keeps a circuit network cookbook of standard patterns. When you hit a control problem, the odds are good that someone has already solved it there, so reach for a known recipe before inventing one.

What you can build with it

The everyday uses are practical, not exotic. Switch off a pump when a tank is full so you stop wasting a fraction. Page a train only when a station actually needs material, so trains idle instead of clogging. Enable or disable a whole train station by signal so the network routes itself. Build a counter or an index to drive a more complex behavior. Each of these replaces a thing you would otherwise babysit, which is the entire value of the system.

Where it fits in a run

Circuits are optional for finishing the game but essential for tight, self-regulating designs, so most players meet them when a factory grows past the point of hand-tuning. They pair naturally with trains, where station enabling and disabling keeps trains from running empty, and with fluids, where pump control prevents overflow. Treat them as a tool you adopt when a specific control problem appears, not a prerequisite you must master up front.

Watch out for

• Reaching for combinators before you have a problem they solve. A circuit you do not need is just complexity.
• Building a clever circuit you cannot debug later. Keep early control logic as simple as the job allows.
• Forgetting that red and green wires are separate channels, which is both a feature and a common source of confusion.

Related: Trains and Oil Processing and Fluids.