Factorio Circuit Network Basics: 5 Circuits That Run Your Base Without You

Your factory doesn’t stop running just because you’re 40 hours into a Vulcanus outpost and forgot to check on Nauvis. Ammo chests run dry. Oil tanks back up into pipes that stall production. A train limit set for early-game traffic chokes a rail network that’s since tripled in size. None of it needs a player watching — it needs five circuits.

Circuit network tutorials usually teach it as a grab-bag: chest limiters, oil pumps, alarm lamps, ten more examples, pick whatever looks useful. That’s not wrong, it’s just not helpful when you’re staring at a save file and don’t know which five circuits actually matter before you leave a planet unattended — exactly the risk our Space Age beginner’s guide flags when it asks whether your base can survive 10 hours without you.

This guide skips the exhaustive catalog. Five circuits, in the order they stop being optional, each with the exact signal condition, the mechanism behind why it works, and which type of player should build it first. Verified on Factorio 2.0 / Space Age (2.1.x) — combinator behavior changed significantly in the 2.0.7 patch, covered below. Values may shift with future Wube updates; check the official wiki if something here stops matching what you see in-game.

Build Order Before You Touch a Single Wire

• Wire a tank or chest straight to a lamp with a “less than” condition first — your first alarm, no combinator required
• Add a combinator only when a plain wire condition can’t express the logic (oil cracking, backup power)
• Keep alarm networks and control networks on separate wire colors, even when it means running an extra wire
• Set train stop limits from a chest signal before you scale past 3-4 trains sharing a station name
• Build the backup-power latch before an extended AFK trip, not after a brownout wakes you up
• Label every combinator (right-click it, type a name) — a decider named “Ammo Alarm” saves 20 minutes of wire-tracing later
• Re-check every threshold after adding a new production line — a number calculated for one factory size silently mis-fires at another

Circuit Network Fundamentals in 90 Seconds

Wires carry signals, not power. Two colors, red and green, form completely independent networks — if both happen to touch the same power pole, they still don’t merge; a red-wire condition never sees a green-wire signal. Every item and fluid a chest, tank, belt, or train stop can hold becomes a signal automatically, and when multiple senders share a wire their values add together rather than overwrite each other. [1]

Four combinators handle the actual logic. The constant combinator broadcasts up to 20 values you set by hand — a static input, not a sensor. The decider combinator compares two signals and passes a value through only when a condition is met, using one of six operators (>, =, <=, !=); since the 2.0.7 patch, a single decider can chain multiple AND/OR conditions instead of needing several stacked combinators. [5] The arithmetic combinator does math on two signals — 11 operations from addition to bitwise shifts, with division truncating rather than rounding (21 / 10 outputs 2, not 2.1). [6] The selector combinator, new in Space Age's 2.0.7 update, sorts, counts, or randomly samples signals; its Quality Filter and Rocket Capacity modes are Space Age-only. [4]

Four of the five circuits below need nothing but a wire and a condition set on the receiving building. Only two touch a combinator.

Circuit 1: The Low-Resource Alarm

The single highest-value circuit is also the simplest: wire a storage chest or tank straight to a lamp with a “less than” condition, no combinator involved. Set an ammo chest’s condition to “Firearm Magazine < 100" and the lamp lights the moment resupply stalls — the same pattern the official cookbook uses for empty-barrel monitoring. [2]

The mechanism: the chest broadcasts its contents as item-channel signals continuously, and the lamp reads the network and evaluates its own condition every tick — there’s no polling delay to configure. Swap the lamp for a programmable speaker and you get an audio cue instead of a visual one; same wire, same condition, different receiving entity.

New players: start here and stop at one alarm per resource type until it’s obviously not enough. Optimizers: feed the alarm’s output signal into a train stop’s circuit condition (Circuit 3) so a shortage doesn’t just light a lamp — it stops trains from wasting a trip to a station with nothing to load.

Circuit 2: Automatic Oil Cracking

Automatic oil cracking is the first circuit that actually needs a decider, because the condition compares two tank levels instead of one against a constant.

Wire the light oil storage tank to the pump feeding the light-oil-to-petroleum-gas recipe with a red wire, and set the pump’s condition to “Light Oil > 20,000.” [2] Below that threshold the pump stays off and light oil accumulates for lubricant and plastic; above it, the pump activates and starts converting the surplus. Mirror the setup for heavy oil with a second pump reading “Heavy Oil > 20,000” into the heavy-to-light cracking recipe.

The cookbook’s more robust version compares the two tanks directly instead of using fixed constants — condition “Heavy Oil > Light Oil” on the heavy-cracking pump — so the system self-balances as your consumption ratio shifts instead of drifting toward one tank’s cap. [2] That’s the version worth building once your refinery output stops matching a fixed 20,000 threshold.

Casual players: the fixed threshold is fine and takes two minutes. Optimizers: build the comparative version — it never needs revisiting when your ratios change.

Circuit 3: Dynamic Train Limits

A fixed train limit is a symptom, not a fix. A station set to “limit 2” because that’s what worked at 40 science-per-minute chokes the same station at 200 SPM without anyone touching a setting. The fix is a train stop limit driven by what’s actually sitting in storage.

Wire the output chests or tanks at a loading station to the train stop itself, then set the stop’s circuit-controlled limit to read a divided signal — an arithmetic combinator dividing total stored iron plates by 4,000 (a full cargo wagon: 40 slots at a 100-plate stack size) gives a limit that grows and shrinks with supply automatically. [7][8] When storage empties, the limit drops toward zero and trains stop reserving a station with nothing to load; when a backlog builds, the limit rises to clear it faster. [3]

This matters more once you’re running the multi-station networks our Space Age trains guide covers — a static limit that made sense for one cargo route becomes a bottleneck the moment a second route shares the line. One caveat worth knowing before you build this: changing the limit doesn’t recall a train already en route — it finishes its current wait conditions first, so the circuit adjusts future dispatches, not the train currently loading. [3]

Circuit 4: Turret Ammo Resupply Alarm

This is Circuit 1’s alarm pattern applied to the failure mode that ends long-running saves. Laser turrets can’t run dry — they draw power, not ammo — but if any part of your defense line still runs gun or rocket turrets, an empty ammo chest during an unattended biter wave is how a base dies while you’re on another planet.

Wire every ammo chest along a gun-turret line to a single lamp, or to a decider combinator with an OR condition chaining multiple chest signals, using “Firearm Magazine < 50" per chest. Because item signals are additive when several chests share a wire, you can also just sum the whole line's ammo and alarm when the total drops below a base-wide threshold — simpler to wire, less precise about which specific chest is empty.

If your perimeter is fully converted to laser turrets, skip this circuit — it has nothing left to monitor.

Circuit 5: Backup Power with Hysteresis

A power switch wired to “Accumulator charge < 10%" sounds like a complete backup-power circuit, and isn't. Without a second condition, it flips on and off every time charge crosses 10% in either direction, hammering steam engines or backup generators with rapid cycling that wastes fuel for no benefit. [2]

The fix is an SR latch: one decider condition sets the backup on when accumulator charge drops below 20%, a second condition resets it off only once charge climbs back above 90%. [2] The gap between the two thresholds — hysteresis — is what stops the flicker. The system commits to “on” and stays there until charge genuinely recovers, not until it ticks one percent back over a single line.

This is the one circuit here that rewards over-engineering. A backup generator cycling every few seconds during a cloudy stretch burns through solid fuel or nuclear cells for nothing; a latch with a wide enough gap runs it in long, deliberate stretches instead. Build this before an extended AFK trip, not after a brownout wakes you up to a dead defense grid.

Which Circuit Should You Build First?

Not every player needs all five on day one.

If you only have time for one circuit before a long unattended trip, build whichever of Circuit 5 (backup power) or Circuit 4 (turret ammo) matches your last close call — a brownout or an empty ammo chest. Every other circuit on this list degrades output over time; those two end a save outright.

Common Circuit Network Mistakes

Three mistakes account for most broken circuits. Crossing red and green wires on the same power pole expecting them to merge — they don’t, and a condition on one color never sees a signal on the other. [1] Confusing “Each” with “Everything”: Each runs a condition or operation once per distinct input signal individually, which is what balancing a bank of chests needs; Everything checks whether a condition holds across all signals at once, and Anything checks if it holds for at least one — swapping these silently turns a per-item threshold into an all-or-nothing gate. And building on pre-2.0 assumptions from an older guide: the selector combinator didn’t exist before Space Age’s 2.0.7 patch, and deciders couldn’t chain multiple AND/OR conditions in one unit before that same update — a tutorial showing three stacked deciders can now be built as one. [4][5]

Frequently Asked Questions

Do I need a combinator for any of these five circuits?
Only two — the comparative oil-cracking version and the backup-power latch. The alarm, the fixed-threshold oil pump, and the turret ammo alarm run on a wire and a condition set directly on the receiving building. If combinators feel intimidating, build the other three first; you’ll have three working circuits before you need to learn what a decider is.

Should I wire everything to one shared network to save time?
No — it’s the fastest way to get conditions that misfire silently. A shared network sums every signal from every connected device, so a chest reading “Iron Plate” on the same wire as an unrelated iron producer sees a combined total instead of its own contents. Keep alarm networks and control networks on separate wire colors even when it costs an extra wire run.

Will circuit conditions built on an older Factorio version still work?
The comparison operators haven’t changed, but the tools have. The 2.0.7 patch added multi-condition deciders and the selector combinator, so builds designed before Space Age often use more combinators than they need to now. A years-old blueprint is worth re-checking rather than assuming it’s still the leanest way to solve the problem.

What’s the actual difference between a decider and an arithmetic combinator here?
A decider asks a yes/no question and passes a signal through only when the answer is yes — it’s a gate. An arithmetic combinator always outputs something; it transforms a signal instead of filtering it. Four of the five circuits above only need a gate, which is why they never touch an arithmetic combinator at all.

Key Takeaways

Five circuits, roughly two hours of wiring, and the difference is a base that survives an unattended trip instead of one that needs checking every twenty minutes. Start with the alarm, add the oil and train circuits once your factory is big enough to need them, and build the backup-power latch before your next extended AFK — not after a brownout finds the gap in your setup.

Circuits aren’t the only unattended-automation tool Space Age offers. For hands-off logistics and combat instead of hands-off monitoring, our Spidertron guide covers autonomous waypoint routes and multi-planet drone squads.

Sources

• Circuit network — Official Factorio Wiki
• Circuit network cookbook — Official Factorio Wiki
• Train stop — Official Factorio Wiki
• Selector combinator — Official Factorio Wiki
• Decider combinator — Official Factorio Wiki
• Arithmetic combinator — Official Factorio Wiki
• Cargo wagon — Official Factorio Wiki
• Iron plate — Official Factorio Wiki

Michael R.

I've been playing video games for over 20 years, spanning everything from early PC titles to modern open-world games. I started Switchblade Gaming to publish the kind of accurate, well-researched guides I always wanted to find — built on primary sources, tested in-game, and kept up to date after patches. I currently focus on Minecraft and Pokémon GO.