Circuit Simulator Guide
Falstad alternative (2026): When you need real SPICE in a browser
Updated May 2026 · By the CircuitSim team · 8 min read
Falstad Circuit Simulator (also known as circuitjs or the Circuit Simulator Applet) is one of the most beloved free tools in electronics education. Originally written by Paul Falstad as a Java applet and ported to the browser by Iain Sharp using GWT, it has been used in classrooms, in physics labs, and in late-night home tinkering for two decades. The animated current flow, the coloured charge visualisation, the way you can pause and step through a circuit and watch the maths happen, these are why it has lasted.
It is also not a SPICE simulator. Falstad uses what the project itself calls "textbook" component models, designed for real-time visualisation rather than for SPICE-accurate analysis. There is no DC operating point analysis as a separate step, no AC small-signal Bode plot, no .model import for a transistor from a datasheet, and no temperature dependence in the models. If your work or curriculum has moved past intuition and into analysis, Falstad cannot get there. That is when people look for an alternative.
This guide compares Falstad with the alternative we build, CircuitSim, plus the other tools that come up in the same search. Falstad is excellent at what it was built for; this is about what to use when you need something Falstad was never designed to do.
TL;DR, Quick picks
- Best free browser-based alternative with real SPICE: CircuitSim. Free during open beta, runs DC, AC, and transient analysis with a real SPICE engine, plus interactive simulation that captures the live-tweak feel Falstad fans value.
- Best free desktop SPICE if a browser is not required: LTspice. Free, deep component library, Windows-first.
- Best open-source SPICE for scripted workflows: ngspice. The actively-maintained open-source descendant of Berkeley SPICE3.
A note on bias
This guide is published on CircuitSim's website. CircuitSim is a free browser-based SPICE simulator built by the team that publishes this article, so we have a horse in this race. We have done our best to be straightforward about where Falstad is still the right tool (it is, for several real use cases), where CircuitSim is the right step up, and where neither is what you need.
What Falstad does well
Falstad earned its place in the canon of free electronics tools by doing one thing better than almost anything else: making circuit behaviour visible. Voltage is shown by colour, current is shown by animated charges flowing through the wires, and the whole thing updates in real time as you change values. For a first-time learner staring at a series-parallel resistor problem and trying to understand why current divides the way it does, Falstad is a different kind of teacher than a textbook diagram.
The other things Falstad gets right:
- Free in every sense. No subscription, no nag screen, no account, no install. Open source under GPLv2 with the full code on GitHub.
- Active maintenance. Paul Falstad maintains the original at github.com/pfalstad/circuitjs1 and Iain Sharp maintains the browser port at github.com/sharpie7/circuitjs1, with community contributors adding components, translations, and UI improvements over the years.
- Runs offline after first load. Once the page is cached, the simulator runs without an internet connection.
- A library of example circuits. Built-in worked examples covering analog, digital, audio, RF, motors, and active filters. A whole curriculum's worth of starter material.
- Scope graphs and parameter tuning at runtime. Click a probe onto a node, drag a slider on a component, watch the scope trace move. The interaction model is unusually direct.
Best for
Teaching circuit theory at the conceptual level. Physics-class demonstrations of how current and charge actually behave. Hobbyist exploration. Quick sketches where the visualisation matters more than the analytical depth. Anyone who values open-source and zero cost.
Where Falstad stops
The same engine that makes Falstad's animation work also limits what it can do analytically. Real-time visualisation and SPICE-accurate analysis are different design goals, and Falstad's documentation is honest about the choice: "textbook" component models, designed for clarity rather than for production simulation.
Concretely:
- No DC operating point analysis as a discrete step. Falstad shows you the steady state by running the simulation, but there is no formal .op solve that gives you a clean numerical answer to "what is the bias of this transistor at quiescent conditions?"
- No AC small-signal analysis. No Bode plots, no frequency response sweep, no phase margin computation. Filters and amplifiers cannot be characterised the way a SPICE tool characterises them.
- No standard SPICE .model imports. If a datasheet provides a SPICE model for a specific BJT, op-amp, or MOSFET, Falstad cannot use it. Component coverage stays at the level of generic textbook parts.
- No temperature dependence. Diode and BJT junction capacitances, temperature coefficients, and other real-world non-idealities are not modelled.
- No microcontroller or embedded firmware simulation. Arduino, ESP32, and Pi Pico work is not what Falstad is for.
- 2D schematic only. Excellent for circuit theory; not designed to teach the physical layout of a breadboard or PCB.
None of this is a bug. Falstad is built to make circuits visible and intuitive, and it does that beautifully. Expecting SPICE results from it is asking a different question than the one it was designed to answer.
The closest browser-based alternative with real SPICE
CircuitSim Our pick
Yes, this is our tool, and yes, we put it first. With reason: CircuitSim is built specifically to put real SPICE simulation in a browser, with the same analysis types LTspice and CircuitLab users already know, while keeping the live-tweak interaction feel that Falstad fans value.
What works:
- Industry-standard SPICE engine. The simulation kernel is compiled to WebAssembly and runs locally in your browser. DC operating point, AC small-signal, and transient analysis, the same set used in professional EDA tools and the analyses Falstad cannot run.
- Interactive simulation mode. CircuitSim's interactive operating-point mode updates the schematic in real time as you turn a knob on a component, with bias points and live measurements re-solving up to 120 times per second. This is the closest thing to Falstad's live-tweak experience on top of a real SPICE engine.
- Custom interactive components. Sliders, switches, potentiometers, and custom-built parts that respond to live input during simulation. Combined with the custom component creator (paste a .model, define a .subckt, build a fully dynamic part), this gives you the responsive feel of Falstad with SPICE underneath.
- Schematic editor and netlist editor side by side. Standard SPICE syntax pastes in unchanged. .tran, .ac, .dc, .op directives work as-is.
- Free during open beta. No credit card, no install, no time limit.
Where CircuitSim is still catching up:
- Not open source. Falstad is GPLv2 with full source on GitHub. CircuitSim is a commercial product in open beta, free to use, but not open source.
- Smaller built-in component library than SPICE veterans. CircuitSim ships standard SPICE models for common passives, transistors, op-amps, diodes, and logic; the library is expanding monthly toward a 5,000+ target by Fall launch. If a part you need is not in the library, the custom component creator handles it from a .model statement.
- Internet required to load. Falstad runs offline after first load; CircuitSim needs the page to load each session. Once loaded the simulation runs locally in the browser.
- Open beta. SPICE engine and editor are stable. Full launch planned for July or August 2026.
Best for
Students whose curriculum has moved past Falstad's visualisation depth into Bode plots, transistor biasing, and op-amp design. Educators teaching intermediate analog on Chromebooks or in a browser tab. Anyone who wants live-tweak interactive simulation with a real SPICE engine underneath instead of textbook models.
Get started free at circuitsim.com →
Other alternatives worth considering
LTspice Best desktop SPICE
LTspice is Analog Devices' free desktop SPICE simulator. Deep, accurate, hand-tuned over decades. Windows-first with a less capable macOS build, no browser version, no Chromebook. If you can install software and you are on Windows, LTspice is exceptional. If you cannot install software, it cannot help.
ngspice Open source, command-line
ngspice is the actively-maintained open-source descendant of Berkeley SPICE3. Command-line, batch-scriptable, used inside KiCad's simulator and many academic toolchains. The right tool when SPICE is one step in a larger automation pipeline rather than an interactive editor session. Like Falstad, it is open source; unlike Falstad, it is a real SPICE engine.
EveryCircuit Same animated category
EveryCircuit is the polished commercial cousin of Falstad: same real-time animated category, native mobile apps, $14.99 one-time price for the full version (free tier limited to 5-component user circuits). Also not a SPICE simulator. See our EveryCircuit alternative guide for the full comparison.
Tinkercad Circuits K-12 classroom
Tinkercad Circuits from Autodesk pairs a photoreal virtual breadboard with a built-in Arduino simulator. Great for introductory K-12 lessons and beginner Arduino projects; same simulation depth ceiling as Falstad. See our Tinkercad Circuits alternative guide.
Falstad vs the alternatives
| Capability | Falstad | CircuitSim | LTspice | ngspice |
|---|---|---|---|---|
| Runs in a browser | Yes | Yes | No | No |
| Real SPICE engine | No (textbook models) | Yes | Yes | Yes |
| DC operating point, AC analysis | No | Yes | Yes | Yes |
| Animated current flow | Yes (the highlight) | Interactive op mode (120 Hz) | No | No |
| Custom SPICE .model imports | No | Yes | Yes | Yes |
| Temperature dependence and parasitics | No | Yes | Yes | Yes |
| Open source | Yes (GPLv2) | No (free during beta) | No (free) | Yes |
| Runs offline after load | Yes | Internet required to load | Yes | Yes |
| Runs on Chromebook / iPad | Yes | Yes | No | No |
| Price | Free, open source | Free (open beta) | Free | Free |
Move a Falstad circuit to CircuitSim in 3 steps
- Open the Falstad circuit and inspect it. Use File then Export Circuit (or the circuit-text view) to see Falstad's plain-text representation. Note the component values, connections, and any specific source or scope settings.
- Rebuild the circuit in CircuitSim. Open circuitsim.com, start a new circuit, and recreate the schematic. Standard SPICE components map one to one for resistors, capacitors, inductors, sources, diodes, BJTs, and op-amps.
- Run the SPICE analyses Falstad cannot run. Add a DC operating point for clean bias values, an AC sweep for a Bode plot, or a transient analysis with integration step control. These are the analyses that justified the move in the first place.
Frequently asked questions
What is the Falstad Circuit Simulator?
Falstad Circuit Simulator (also known as circuitjs or the Circuit Simulator Applet) is a free open-source browser-based circuit simulator originally written by Paul Falstad as a Java applet and ported to JavaScript by Iain Sharp using GWT. It is published under GPLv2 with source code on GitHub and contributions from a community of developers. It runs in any browser with no install or account, and is best known for its real-time animated visualisation of current flow.
Is Falstad a SPICE simulator?
No. Falstad uses textbook component models designed for real-time animation and visualisation, not the SPICE algorithm. It does not simulate temperature-dependent behaviour, diode or BJT junction capacitances, or other real-world non-idealities. There is no DC operating point analysis as a separate step, no AC small-signal Bode plot, and no .model import for parts from a datasheet. Falstad is an excellent intuition and teaching tool; it is not a SPICE replacement.
What is a free alternative to Falstad that runs real SPICE in the browser?
CircuitSim is a browser-based SPICE simulator that is free during open beta with no credit card and no install. It runs DC operating point, AC small-signal, and transient analysis on standard SPICE component models, accepts .model imports from a datasheet, and includes an interactive simulation mode that updates the schematic in real time as you tweak component values, similar in spirit to Falstad's animated workflow but on top of a real SPICE engine.
When does Falstad still win over CircuitSim?
Falstad is the better fit for animated current-flow visualisation as a teaching aid, for physics-class demonstrations of how current and charge behave, for offline use on a laptop where the simulator runs without an internet connection after the page loads once, and for anyone who values open-source code under GPLv2. The simulator is mature, maintained, and free in every sense. If your need is intuition rather than analysis, Falstad is hard to beat.
Why would I move from Falstad to a real SPICE simulator?
The same reasons people move on from any visualisation tool: a curriculum or project needs analysis Falstad cannot do. Examples include drawing a Bode plot for a filter, computing a transistor bias point that matches a textbook calculation, simulating an op-amp circuit with a SPICE model from a manufacturer datasheet, or running a transient analysis with precise integration step control. Once those become the work, a SPICE tool is the right next step.
Can I import a Falstad circuit into CircuitSim?
Not directly. Falstad uses its own plain-text save format and does not export to SPICE netlists. The practical migration is to open the Falstad circuit, capture the topology and component values, and rebuild the schematic in CircuitSim using the corresponding standard SPICE components. Most teaching-scale circuits take ten to fifteen minutes to recreate.
Is Falstad still being maintained?
Yes. Paul Falstad maintains the original code at github.com/pfalstad/circuitjs1, and Iain Sharp maintains the actively-developed browser port at github.com/sharpie7/circuitjs1. The project has a community of contributors who have added components, translations, and UI improvements over the years. The licence is GPLv2.
Is CircuitSim really free, or is there a paid tier coming?
CircuitSim is free during open beta with no credit card and no time limit. A full launch is planned for July or August 2026 with paid tiers for institutions and educators after launch; a free individual tier will continue. We will give plenty of notice before anything changes for existing users.
Sources
Falstad Circuit Simulator is the work of Paul Falstad and contributors, published under GPLv2. The browser port (circuitjs1) is maintained by Iain Sharp. LTspice is a trademark of Analog Devices, Inc. EveryCircuit is a trademark of MuseMaze, Inc. Tinkercad and Autodesk are trademarks of Autodesk, Inc. ngspice is maintained by the ngspice project. CircuitSim® is a registered trademark of Elsewhere Labs LLC. CircuitSim is not affiliated with any of the above. All other marks belong to their respective owners.