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CircuitJS1 is an electronic circuit simulator that runs in the browser. It was originally written by Paul Falstad as a Java Applet. It was adapted by Iain Sharp to run in the browser using GWT.

For a hosted version of the application see:

Thanks to: Edward Calver for 15 new components and other improvements; Rodrigo Hausen for file import/export and many other UI improvements; J. Mike Rollins for the Zener diode code; Julius Schmidt for the spark gap code and some examples; Dustin Soodak for help with the user interface improvements; Jacob Calvert for the T Flip Flop; Ben Hayden for scope spectrum; Thomas Reitinger, Krystian Sławiński, Usevalad Khatkevich, Lucio Sciamanna, Mauro Hemerly Gazzani, J. Miguel Silva, and Franck Viard for translations; Andre Adrian for improved emitter coupled oscillator; Felthry for many examples; Colin Howell for code improvements. LZString (c) 2013 pieroxy.

Building the web application

The tools you will need to build the project are:

  • Eclipse, Oxygen version.
  • GWT plugin for Eclipse.

Install "Eclipse for Java developers" from here. To add the GWT plugin for Eclipse follow the instructions here.

This repository is a project folder for your Eclipse project space. Once you have a local copy you can then build and run in development mode or build for deployment. Running in super development mode is done by clicking on the "run" icon on the toolbar and choosing from the "Development Mode" tab which appears. Building for deployment is done by selecting the project root node and using the GWT button on the Eclipse taskbar and choosing "GWT Compile Project...".

GWT will build its output in to the "war" directory. In the "war" directory the file "iframe.html" is loaded as an iFrame in to the spare space at the bottom of the right hand pannel. It can be used for branding etc.

Deployment of the web application

  • "GWT Compile Project..." as explained above. This will put the outputs in to the "war" directory in the Eclipse project folder. You then need to copy everything in the "war" directory, except the "WEB-INF" directory, on to your web server.
  • Customize the header of the file "circuitjs1.html" to include your tracking, favicon etc.
  • Customize the "iframe.html" file to include any branding you want in the right hand panel of the application
  • The optional file "shortrelay.php" is a server-side script to act as a relay to a URL shortening service to avoid cross-origin problems with a purely client solution. You may want to customize this for your site. If you don't want to use this feature edit the file before compiling.
  • If you wish to enable dropbox loading and saving a dropbox API app-key is needed. This should be edited in to the circuitjs.html file where needed. If this is not included the relevant features will be disabled.

The link for the full-page version of the application is now: http://<your host>/<your path>/circuitjs1.html (you can rename the "circuitjs1.html" file if you want too though you should also update "shortrelay.php" if you do).

Just for reference the files should look like this

-+ Directory containing the front page (eg "circuitjs")
  +- circuitjs.html - full page version of application
  +- iframe.html - see notes above
  +- shortrelay.php - see notes above
  ++ circuitjs1 (directory)
   +- various files built by GWT
   +- circuits (directory, containing example circuits)
   +- setuplist.txt (index in to example circuit directory)


You can link to the full page version of the application using the link shown above.

If you want to embed the application in another page then use an iframe with the src being the full-page version.

You can add query parameters to link to change the applications startup behaviour. The following are supported:

.../circuitjs.html?cct=<string> // Load the circuit from the URL (like the # in the Java version)
.../circuitjs.html?ctz=<string> // Load the circuit from compressed data in the URL
.../circuitjs.html?startCircuit=<filename> // Loads the circuit named "filename" from the "Circuits" directory
.../circuitjs.html?startCircuitLink=<URL> // Loads the circuit from the specified URL. CURRENTLY THE URL MUST BE A DROPBOX SHARED FILE OR ANOTHER URL THAT SUPPORTS CORS ACCESS FROM THE CLIENT
.../circuitjs.html?euroResistors=true // Set to true to force "Euro" style resistors. If not specified the resistor style will be based on the user's browser's language preferences
.../circuitjs.html?usResistors=true // Set to true to force "US" style resistors. If not specified the resistor style will be based on the user's browser's language preferences
.../circuitjs.html?running=<true|false> // Start the app without the simulation running, default true
.../circuitjs.html?hideSidebar=<true|false> // Hide the sidebar, default false
.../circuitjs.html?hideMenu=<true|false> // Hide the menu, default false
.../circuitjs.html?editable=<true|false> // Allow circuit editing, default true
.../circuitjs.html?positiveColor=%2300ff00 // change positive voltage color (rrggbb)
.../circuitjs.html?negativeColor=%23ff0000 // change negative voltage color

Building an Electron application

The Electron project allows web applications to be distributed as local executables for a variety of platforms. This repository contains the additional files needed to build circuitJS1 as an Electron application.

The general approach to building an Electron application for a particular platform is documented here. The following instructions apply this approach to circuit JS.

To build the Electron application:

  • Compile the application using GWT, as above.
  • Download and unpack a pre-built Electron binary directory version 9.3.2 for the target platform.
  • Copy the "app" directory from this repository to the location specified here in the Electron binary directory structure.
  • Copy the "war" directory, containing the compiled CircuitJS1 application, in to the "app" directory the Electron binary directory structure.
  • Run the "Electron" executable file. It should automatically load CircuitJS1.

Known limitations of the Electron application:

  • "Create short URL" on "Export as URL" doesn't work as it relies on server support.
  • For diodes, "Create Simple Model" doesn't work as it relies on a javascript feature that is not supported.

Thanks to @Immortalin for the initial work in applying Electron to CircuitJS1.


This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.


Electronic Circuit Simulator in the Browser







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