Falstad circuit simulator: see electronics work in your browser
Falstad's circuit simulator animates voltage and current in a circuit you can edit in your browser. Its GitHub page calls it free software under the GPL, and we saw no price or login.
What it is
Falstad's circuit simulator (CircuitJS1) draws an electronic circuit on screen and shows it working. Its page says: "When the applet starts up you will see an animated schematic of a simple LRC circuit." Green means positive voltage, grey is ground, red is negative, and moving yellow dots show current. You can click a switch, or right-click a part and choose Edit to change its value. It runs in the browser. It was written by Paul Falstad and adapted for the browser by Iain Sharp, per its GitHub page. It is not an AI tool. We included it as a free tool for learning electronics.
Is it free?
We saw no price, login or paid version. The GitHub page says the program "is free software" under the GNU General Public License, version 2 or later, and lists the licence as GPL v2. That is the project's own wording. The falstad.com page itself does not mention a price.
What you can do with it
- Try ready-made circuits. The page says the Circuits menu has "a lot of sample circuits". Falstad also has an index of electronics demonstrations: Ohm's law, resistors, capacitors, inductors, filters, transformers, diodes and more.
- Change a circuit and watch what happens. Edit a part and the animation updates, so you can see how voltage and current react.
- Use it in a browser or offline. The page links a full-screen version and standalone (offline) versions. The GitHub page mentions a desktop (Electron) app with known limitations.
- Add a microcontroller. The page links an "AVR8js version". We did not try it.
What to watch out for
- It is a circuit simulator, not a hardware kit. The page also links an AVR8js version, but we did not check its microcontroller features.
- The interface looks old. The page still calls it an "applet" and refers to the original Java version. We did not test how it behaves on a phone.
- Hosted in more than one place. GitHub lists Paul Falstad's site, Iain Sharp's site and a development branch. Use the falstad.com address on this page.
Privacy and data
- We did not find a privacy policy on falstad.com.
- The page shows no sign-in. We did not run the simulator to see whether it saves or sends anything, and we did not check whether the site uses analytics.
India
- It is a normal website with no region limit on the pages we read. No rupee price is involved.
Frequently asked questions
Is the Falstad circuit simulator free?
The project describes itself as free software under the GPL, and we saw no price or login.
Do I need to install anything?
No. It runs in the browser. Offline versions are also linked.
Can I save my circuit?
The GitHub page credits file import and export work. We did not test saving.
Sources: Falstad Circuit Simulator, Electronics demonstrations, CircuitJS1 on GitHub. Checked 16 October 2026.
Independent guide, not affiliated.
Scoped recheck 23 October 2026: falstad.com circuit page and the CircuitJS1 GitHub project. The page still describes the animated LRC schematic, and the project README still describes the program as free software under the GPL (version 2 or later). Added the official app icon from the source repository; no other fact changes. Other claims retain their original source-check date. Original publication date unchanged.