Falstad

Falstad is the Falstad Circuit Simulator, an interactive tool for creating and simulating electrical circuits. Developed by Paul Falstad, it enables real-time visualization of how circuits behave, making it a popular resource in education and self-study. The original simulator began as a Java applet and ran in browsers that supported Java. In response to changes in browser security and plugin support, a JavaScript port called CircuitJS was released, allowing use directly in modern web browsers without plugins. The online version is widely accessible at Falstad’s website, and the code has been published and ported by the community.

The simulator models circuits using a real-time time-domain solver. Users place symbols for components such as resistors, capacitors, inductors, diodes, transistors, and operational amplifiers, connect them with wires, and apply sources. Wires and nodes are color-coded to reflect voltage and current, and the program can display transient responses, waveform graphs, and other diagnostics. It supports switches, pulse sources, AC analysis, and a variety of built-in example circuits, including RC charging, amplifiers, oscillators, and digital logic demonstrations.

Falstad is commonly used in classrooms, tutorials, and online demonstrations to illustrate fundamental concepts in electronics, such as Ohm’s law, RC time constants, and feedback. While not a full SPICE replacement, it offers an accessible, interactive platform for exploring circuit behavior and experimenting with designs.