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

Three 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. Decision combinators apply a comparison and output a result when the condition holds. 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.