Falstad CircuitJS is a browser-based, real-time analog circuit simulator that lets you build, probe, and animate electronic circuits without installing any dedicated software — and it ships as a native Mac app for offline use.
What is Falstad CircuitJS?
Falstad CircuitJS is an open-source circuit simulation environment originally written by Paul Falstad, rebuilt in JavaScript so it runs in any modern browser and, via an Electron-style wrapper, as a standalone macOS application. Drop resistors, capacitors, transistors, op-amps, and logic gates onto a schematic canvas, hit Run, and watch current flow in animated, colour-coded streams — live, at whatever speed your Mac can push.
What sets it apart from heavier SPICE tools is immediacy. There is no netlist compilation step, no lengthy solver setup, no licensing wizard. You wire something up and it simulates. The feedback loop is almost physical — tweak a resistor value mid-run and every voltage and current in the circuit updates in real time. I have used it to sanity-check filter designs, debug op-amp gain stages, and walk junior engineers through why their bypass capacitor placement matters, all without leaving a browser tab.
What does Falstad CircuitJS do best?
Real-time animated simulation is where CircuitJS is genuinely unmatched in its class. Most SPICE-family tools run a batch solve and then let you plot waveforms after the fact. CircuitJS runs the solver continuously while you interact, drawing animated electron-flow arrows so the circuit feels alive rather than theoretical.
- Instant component placement — a right-click menu covers passives, semiconductors, digital gates, motors, and basic RF elements in seconds.
- Live scope overlay — click any wire or node to open an oscilloscope-style graph without leaving the canvas.
- Editable while running — change values, swap components, or rewire connections without stopping the simulation.
- Shareable circuits — circuits export as URL-encoded strings or compact text, making it trivial to paste a circuit into a Slack message or GitHub issue.
- Huge example library — the built-in gallery covers AM receivers, PWM controllers, flip-flops, phase-locked loops, and dozens more ready-to-dissect schematics.
For teaching or quick prototyping it is genuinely hard to beat. I have explained switch-mode power supplies to clients by literally dragging the duty cycle slider while the inductor current waveform updated live on screen — no slides required.
Is Falstad CircuitJS free?
Yes — CircuitJS is completely free and open-source, released under the MIT licence. The web version at falstad.com costs nothing. The Mac wrapper is equally free to download. There is no paid tier, no feature unlock, and no subscription.
Who should use Falstad CircuitJS?
CircuitJS earns a place on the Mac of anyone who touches electronics, from students encountering Ohm's Law for the first time to hardware engineers who need a fast sanity check before pulling up LTspice or KiCad. Its visual immediacy makes it exceptional for teaching — the animated current flow is the single best explanation of how a transistor switches that I have ever shown someone.
It is not a substitute for full SPICE when you need accurate parasitics, SPICE model libraries for specific ICs, or PCB-level simulation. For that, LTspice, Qucs-S, or a commercial tool like Keysight ADS is the right call. But for schematic-level intuition and rapid what-if exploration, CircuitJS is faster than any of them.
What are the best Falstad CircuitJS alternatives?
The closest free Mac alternative is LTspice, now available natively on Apple Silicon via Analog Devices — it has a vastly deeper component library and proper SPICE accuracy, but the UI dates to 1999 and there is no live animation. Qucs-S (Quite Universal Circuit Simulator) covers RF and microwave work that CircuitJS cannot touch. For pure digital logic, Logic Friday or the web-based CircuitVerse are lighter-weight options. Multisim Live from NI offers a polished cloud UI but sits behind a subscription wall. For quick, free, animated analog intuition on a Mac, nothing touches CircuitJS.
How does Falstad CircuitJS compare to LTspice?
LTspice wins on simulation fidelity, model depth, and professional credibility — it is what engineers use to sign off on a design before tape-out. CircuitJS wins on speed-to-insight and approachability. LTspice asks you to write or import a netlist and set up transient or AC analysis parameters before you see anything move. CircuitJS starts animating the moment you connect two components. For learning concepts or communicating ideas to non-specialists, CircuitJS is the right tool; for verifying that your 48 V buck converter stays in regulation across temperature, reach for LTspice.
