Write, simulate, and auto-grade ladder logic in any browser. IEC 61131-3, Allen-Bradley, and Siemens dialects — no install, no trial clock.
The short answer
Yes — you can learn ladder logic completely free, in a browser, without buying a PLC or installing vendor software. Contacts, coils, seal-in rungs, timers, and counters behave identically in a scan-cycle simulator, and auto-graded test cases tell you whether your program actually works. The free tier here never expires and needs no credit card.
What you cannot skip is deliberate practice: reading a spec, writing the rung, running the grader, and fixing what failed. The six-step path below sequences the free graded scenarios on this site in the order the concepts build — the same order our paid curriculum uses. Reference pages for ladder logic symbols and worked ladder logic examples are free too.
The path
Every scenario linked below is on the free tier and auto-graded. Budget a few short sessions per step and don't move on until the grader shows all tests green.
Wire one normally open contact to one coil and toggle the input while the simulator runs. Feeling the scan cycle respond is the whole lesson.
Switch → light: one rung, graded against test cases. On our platform this is where everyone starts — about 80% of completed attempts pass it.
Series contacts (AND), parallel branches (OR), and normally closed logic — the three moves that make up most real interlocks.
Motor start-stop with a seal-in branch — the first rung that forces you to think in scan cycles instead of straight lines. Expect to fail it a few times; the grader tells you exactly which test broke.
Motor start-stop scenario → · Latch basics · Latch with stop
Delays and part-counting are the backbone of real machine sequences — and where the grader’s timing test cases start to matter.
A full timed sequence combining everything above. Create a free account (no credit card) to save your programs and track which steps you’ve passed.
The editor — runs in your browser tab
Motor start-stop with seal-in — auto-graded test cases show pass/fail per test
What you get
How it works
The simulator runs a scan-cycle engine in your browser tab — the same read-inputs, execute-program, write-outputs loop a real PLC runs. Your ladder program is compiled to an instruction set that the engine executes rung-by-rung, updating coil states and timer accumulators each scan. Animated I/O indicators update in real time so you can see exactly which contacts are energised at each step.
The auto-grader injects predefined input states and verifies that your outputs match the expected values. Each test case reports the exact mismatch — “Motor output was OFF, expected ON at T=500ms” — so you know which rung to fix. This is qualitatively different from watching a simulation: you get scored feedback on whether your program is actually correct.
The IEC 61131-3 core set: XIC (normally open contact), XIO (normally closed contact), OTE (output coil), OTL/OTU (set/reset coils), TON, TOF, TP (timers), CTU/CTD (counters), comparison instructions (GT, LT, EQ), and arithmetic blocks. The Allen-Bradley dialect adds tag-based addressing; the Siemens dialect adds absolute and symbolic address styles.
The simulator has run entirely in the browser since day one. There is no plugin, no download, and no trial expiry. It works on Chrome, Firefox, Safari, and Edge — on Windows, Mac, Linux, and Chromebook. If you can open this page, you can run the simulator.
No credit card. No install. No trial clock. The free tier does not expire.
From our grading data
These figures come from our own simulator's aggregate grading logs as of 10 July 2026 — not estimates.
10,272
auto-graded ladder logic attempts run on the simulator to date.
80%
of completed graded runs pass the first scenario (switch → light): 2,116 passes out of 2,634 completed runs. Starting really is easy.
19%
of completed graded runs pass the motor start-stop seal-in scenario (127 of 679). The latch is where ladder logic stops being obvious — plan to iterate.
7 in 10
recorded first passes happen on the very first graded run (1,493 of 2,149 first-passes reached within ten runs) — instant grading feedback shortens the loop.
“The seal-in rung is where ladder logic clicks. Fewer than one in five completed attempts at motor start-stop pass — not because it's hard, but because it's the first rung where you have to think in scan cycles instead of straight lines. Free practice with instant grading is exactly how you get through that wall.”