Falstad Circuit Simulator — The Complete Guide: Watch Current Flow Through a Circuit, Right in the Browser

Falstad Circuit Simulator is a circuit simulator that runs in the browser, with no installation and no registration. Instead of a table of numbers it shows current as dots moving along the wires and voltage as colour, so you see at once what the circuit is doing. The browser version is open source under the GPL.

Falstad Circuit Simulator

Paul Falstad

An interactive circuit simulator that runs in the browser, with animated current.

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What you get

Instant feedback — change a resistor value or flip a switch and the circuit updates in real time.
Analogue and digital parts — resistors, capacitors, inductors, diodes, transistors, op-amps and logic gates.
Example library — filters, rectifiers, oscillators, switching supplies and logic circuits, ready to open.
Built-in scope — voltage and current plots for any component, next to the circuit.
Sharing by link — a circuit is saved as text or as a link you can send to a student or colleague.

Who it suits

Teachers and lecturers who want to demonstrate a principle in class, students checking their understanding, and engineers who want to test an idea in a minute. It is not a precision design tool: it has no detailed manufacturer models as SPICE does, and it does not replace proper analysis before production.

Related tools

For accurate simulation with models of real parts — LTspice or ngspice. For circuits with microcontrollers running code — SimulIDE. For digital logic only — Logisim Evolution.