I compared my logic simulator to 9 others. Here is where mine loses

I have written four posts about Boolflow now — the first release, v2, v3 and v4. Every one of them was about what I added.

None of them answered the question I actually get asked, which is some polite variant of: Logisim exists. Why does this?

For a long time I did not have a real answer, because I had never done the work. So I did it. I spent a couple of weeks with nine other digital logic simulators — installing them, building the same handful of circuits in each, reading their docs and their source where it was available — and then I put my own tool in the same table and looked at the result.

The result is not flattering in the places you would expect, and it is better than I thought in one place I did not expect. Both halves are below.

The field

There is no single "logic simulator market". There are three clusters that barely compete with each other, plus one commercial outlier.

Desktop, free, open source. Logisim Evolution is the default answer in most university courses — GPL-3.0, Java, 7.5k stars, and still actively developed (v4.1.0 landed in February 2026). Digital by Helmut Neemann is the connoisseur's pick: GPL-3.0, 5.9k stars, and in several areas — test cases, circuit analysis, hardware export — genuinely ahead of Logisim. LogicCircuit is a free open-source Windows/.NET tool with unrestricted circuit hierarchy, multi-bit buses, a built-in oscilloscope and — since 2024 — HDL export to Verilog and Nand2Tetris. Deeds comes out of the University of Genoa and is the most curricular of the four — combinational and sequential design, an FSM editor, and a microcomputer emulator with a custom 8-bit CPU, all wrapped in course material. Windows only.

Browser, free. CircuitVerse is the serious one: MIT-licensed, subcircuits, timing diagrams, propagation delay control, Verilog modules, testbenches, and a full classroom layer with groups, assignments, deadlines, grading and grade export. Falstad's CircuitJS1 is GPL-2.0 and is really an analog simulator with digital parts — the animated current flow is unmatched for intuition, but it is not organised around gate-to-CPU teaching. simulator.io is clean and account-free to try, has real-time collaboration, and has a 2015 copyright line in its footer that tells you most of what you need to know about its development pace.

Commercial. Logicly is Windows and macOS, aimed squarely at teachers, with custom ICs and step-by-step propagation debugging. It costs $59 for a student licence, $599 for a classroom and $1,299 for a campus — one-time, not subscription.

And then there is DigiSim, which I found late and which is the closest thing to a direct competitor Boolflow has. Browser-based, real-time, four labs (digital, relay, electricity, analog), 61 components, narrated lessons in seven languages, freemium — the free tier gives you 16 of 61 components, Pro is $6/month, classroom is $4/seat/year. It is doing the same thing I am doing — putting the lessons inside the simulator — with a business model and more languages. Finding it was the least fun afternoon of this whole exercise, and also the most useful.

Boolflow, for the table below: browser-only, free, no account, 74 element types, 20 worked examples, exports Verilog / VHDL / C++, generates truth tables, four design layers linked by VIA nodes, and ships 17 theory articles in English, Russian and German.

The table

The bold entries in my own row are the ones that matter. Three of them are gaps.

Where Boolflow loses

I want to be specific here rather than doing the humble-brag thing where the weakness is secretly a strength.

1. No sub-circuits. This is the big one.

Every serious tool in that table lets you build a circuit, save it, and drop it into another circuit as a single block. Logisim, Digital, LogicCircuit, Deeds, CircuitVerse, Logicly, DigiSim — all of them. It is the feature that takes you from "I built a full adder" to "I built a 4-bit ALU out of four full adders" to "I built a CPU".

Boolflow does not have it. What Boolflow has is layers — up to four independent sheets connected by VIA nodes — and I have caught myself describing them as if they solve the same problem. They do not. Layers are a way to stop one canvas becoming unreadable. Sub-circuits are a way to build abstractions. One is a viewing convenience, the other is the thing computer architecture is made of.

Writing the v4 article series taught digital logic. Writing this comparison taught me that my tool tops out exactly where the interesting part of the subject begins.

2. No timing diagrams.

Logisim has a chronogram. Digital has measurement graphs. LogicCircuit has an oscilloscope. CircuitVerse has timing diagrams with configurable propagation delay. Falstad has a scope.

Boolflow shows you the current state of every wire, and nothing else. For combinational logic that is genuinely fine — the truth table generator covers it. For anything clocked, it is a real handicap. You cannot debug a shift register by staring at its present value. I wrote an entire article about metastability for a tool that cannot draw you a single waveform, which in retrospect is funny.

3. No test framework, no classroom layer.

Digital lets you write test cases and run them against a circuit. CircuitVerse has testbenches where an author declares input vectors and expected outputs, plus groups, assignments with deadlines, grading and grade export.

Boolflow has a Circuit Check — a linter that finds unconnected inputs and floating outputs — and that is the whole of its verification story. If you are a teacher with thirty students, CircuitVerse solves your actual problem and Boolflow does not solve any part of it.

4. No path to hardware.

Logisim Evolution has FPGA board integration. Digital goes further: JEDEC output for GAL16v8 and GAL22v10, ATF150x CPLD support up to 128 macro-cells, direct support for BASYS3 and TinyFPGA BX boards, and it will drive ghdl or Icarus Verilog to simulate its own generated HDL.

Boolflow emits Verilog and VHDL text and then wishes you luck. That is a real feature — it is not nothing to get synthesisable HDL out of a drawing — but "here is a file" and "here is your circuit running on a board" are different products.

5. Nobody has heard of it.

Logisim Evolution has 7,482 stars and a decade of course syllabuses pointing at it. Digital has 5,943. Boolflow has me. When a tool is used by thousands of students, the bugs that matter have been found. Mine have not.

Where Boolflow actually wins

Shorter list, and I checked each one against the other nine rather than assuming.

Zero friction, all the way down

Logisim and Digital need Java. LogicCircuit and Deeds need Windows. Logicly needs $59. simulator.io and CircuitVerse need an account before your work survives a browser refresh. DigiSim's free tier caps you at 16 components.

Boolflow: open the URL, place gates, save a JSON file. No install, no login, no tier, no component limit. Only Falstad matches that, and Falstad is an analog simulator you can coerce into doing logic.

That sounds like a small thing until you watch someone try to use a simulator during a forty-minute class on a locked-down school laptop.

C++ export

Nine other tools. Verilog appears in four of them, VHDL in three. C++ appears in none of them.

I added it almost as a curiosity — take the drawn circuit, emit a plain C++ function that computes the same outputs, with an optional test harness. It turns out to be the bridge that people who write software but not hardware actually want. You do not have to learn an HDL to check that the thing you drew does what you think it does; you compile it and call it.

I did not know this was unique until I went looking. It is now the feature I would lead with.

The theory sits next to the tool

This was the whole point of v4 and it holds up. Seventeen articles, each one ending in a link that drops you into the editor to build the thing you just read about.

Only two other tools in the table do this. Deeds does it well and is Windows-only 32-bit software from a university. DigiSim does it better than I do — 98 narrated lessons against my 17 articles — and puts most of them behind a subscription. CircuitVerse, the strongest browser tool by a distance, has documentation rather than a curriculum.

So the niche is real. It is just narrower than I thought and I am not alone in it.

Trilingual, including the interface

Boolflow's UI, its 17 articles and its per-element help all exist in English, Russian and German. Digital has seven interface languages but is desktop Java. DigiSim has seven languages and charges for most of the lessons. CircuitVerse is English-first. Logisim's teaching material is overwhelmingly English.

If you are learning digital logic in German or Russian and you want the theory and the tool in your own language, in a browser, for free, the list of options is short.

The Circuit Check

I could not find an equivalent in any of the other nine as a first-class feature: a button that scans the schematic and tells you which inputs are unconnected and which outputs go nowhere.

It is not clever. It is about eighty lines of code. It is also the single most common thing a beginner gets wrong, and every other tool makes them find it by wondering why nothing happens.

What this changed on my roadmap

I went in wanting to write a piece about how Boolflow compares. I came out with a re-ordered backlog, which is a better outcome.

1. Sub-circuits. Not "some day". Next. Everything else on this list is smaller than the gap between having them and not.
2. Waveforms. A timing panel for clocked circuits. The flip-flop and counter articles are currently writing cheques the editor cannot cash.
3. Test cases. Digital's model — declare inputs and expected outputs, run, see pass or fail — is the right one, and it is also the missing half of "exercises with checkable answers", which I promised in the v4 post.
4. Everything classroom-shaped: deliberately not. Accounts, groups, grading, dashboards. CircuitVerse has done this properly for years and I am not going to do it better as one person. If you need it, use CircuitVerse. I would rather be the tool a single person opens at 2am than a worse version of a platform that already exists.

So which one should you use?

Honestly:

• Taking a university digital logic course? Logisim Evolution. It is what your assignments assume.
• Teaching a class of thirty? CircuitVerse. Nothing else free gives you assignments and grading.
• Pushing a design onto real hardware? Digital. JEDEC, CPLD and board support that nothing else free comes close to.
• Trying to understand what current actually does? Falstad. Watch the dots move.
• Want a paid, polished, teacher-first desktop app? Logicly.
• Want narrated lessons across digital, analog and relay logic and don't mind paying? DigiSim.
• Want to open a tab right now, place a gate within ten seconds, generate a truth table, and read the theory in the same site in English, Russian or German — with no install, no account and no limit? That one is Boolflow.

That last sentence is a narrow claim. It is also, after two weeks of checking, a true one.

Tell me what I got wrong

Genuinely — this is the part I want feedback on:

• If you maintain or teach with one of these tools and I have misrepresented it, say so and I will correct the post. I tested each one myself but two weeks is two weeks.
• If you use a simulator I did not cover, name it. I deliberately left out Multisim, Proteus, Tinkercad and Wokwi as different categories, but I may have missed something in-category.
• Is my read on sub-circuits right? Is that the feature that would actually change whether you'd use Boolflow, or am I fixating on it because it is the most obvious hole?
• And the C++ export: is that useful to you, or is it a solution I invented for a problem nobody has?

Boolflow is at boolflow.site, the articles are at boolflow.site/articles, and it is free with nothing to install.