Simulator

Online circuit simulator

Build a circuit and watch it run: animated current, live voltages, oscilloscopes, logic, motors and more. Free, with no account and no component limit.

Tip: right-click (or long-press) to add parts, drag to wire, and use Circuits in the menu for 360+ examples. The simulator is Falstad CircuitJS1, free software under the GNU GPL.

Example circuits to open

Start here

Power and rectifiers

Filters and AC

Transistors and op-amps

Timers and logic

What you can simulate

The simulator solves your circuit many times a second and animates it, so you see current move, capacitors charge and transistors switch. It covers far more than a phone app: resistors, capacitors and inductors; diodes, LEDs and Zeners; BJTs, MOSFETs and JFETs; op-amps and comparators; transformers, relays and motors; three-phase sources; logic gates, flip-flops, counters and the 555 timer; plus voltmeters, ammeters, an ohmmeter and multi-channel oscilloscopes with FFT. Custom subcircuits let you package a design as your own chip.

For electricians, not only for electronics

Three-phase sources, motors, relays and transformers mean you can try out control circuits and power ideas safely: see what an open neutral does to a 120/240 V split-phase supply, or how a capacitor changes a motor’s current. We are building a library of electrician-focused circuits (starters, switching, ladder logic) and linking every calculator here to a ready-made circuit.

Saving and sharing

Circuits save in your browser, and File › Export as link makes a URL that opens the exact circuit for anyone, no account needed. Nothing is uploaded unless you choose to share it.

Coming next: SPICE analysis

Real-time simulation is perfect for learning and quick checks. For design work we are adding an Analyze mode built on ngspice, the open-source SPICE engine, with operating-point tables, DC sweeps, AC/Bode plots, noise and Fourier analysis, and netlist import and export.

Questions people ask

Is this circuit simulator really free?

Yes. There is no account, no trial and no limit on the number of components or circuits. It runs entirely in your browser.

Does it work on a phone?

Yes, though a tablet or laptop is easier for building larger circuits. Opening and running example circuits works well on phones.

Is this an alternative to Multisim Live or EveryCircuit?

For most learning and quick design checks, yes: it simulates analog and digital parts live, with scopes and meters, and has no component cap. For SPICE-style analysis, our ngspice-based Analyze mode is on the way.

Who made the simulator?

It is CircuitJS1 by Paul Falstad and Iain Sharp, free software under the GNU General Public License. We host it unmodified and publish the source and licence.