Pick a part, click to place it, drag between the dots to wire — the circuit runs instantly. Watch current flow, glowing LEDs, spinning motors and live voltages. 8 guided lessons, 11 examples, share links. Free, no signup, works on phones.
Nothing to install, nothing to sign up for.
From an empty board to a glowing LED in under a minute.
Click a part in the bin, then click the board. Press R to rotate while placing.
Drag from one terminal dot to another. Wires bend at right angles and a dot marks every junction.
Current dots start moving. Click switches, hold push buttons and drag a part's value slider.
Hover or select a part for voltage, current and power, or open the scope to plot them over time.
Follow the 8 lessons or load one of 11 examples, then copy a share link or export a PNG.
R = (Vbattery − VLED) ÷ I4k7, 10k or 100u instead of long numbers.Circuit Lab is a free circuit simulator that runs entirely in your browser. Every time something changes it rebuilds the circuit as a set of equations from Kirchhoff’s laws and solves them, so the numbers you read are calculated, not looked up. Wires and parts are coloured by the voltage at each point, and the moving dots show how much current flows where.
Every part shows Ohm’s law with your real numbers, resistor colour bands, whether an LED is under- or over-driven and what resistor to use instead.
Push too much current through an LED and it burns out; a short circuit trips a warning and blows fuses. Press “replace” and try again — nothing lost.
Eight bite-size challenges, from lighting an LED to fixing a two-transistor blinker, with hints and automatic goal detection.
Modified Nodal Analysis with Newton–Raphson — the same approach as SPICE — so diodes, LEDs and transistors behave non-linearly and capacitors charge on a real curve.
Tap to place, drag to wire, pinch to zoom. Works on phones, tablets, Chromebooks and projectors.
No account, no ads, no upload. Circuits save in your browser and share as a link that contains the whole circuit.
| Circuit Lab | Falstad CircuitJS | Tinkercad Circuits | PhET DC Kit | |
|---|---|---|---|---|
| Signup needed | No | No | Yes (Autodesk) | No |
| Animated current flow | Yes | Yes | No | Yes |
| Transistors, capacitors, AC | Yes | Yes, hundreds of parts | Yes + Arduino | No (DC kit) |
| Guided lessons with goal checks | 8 built in | No | Separate lesson plans | Teacher activities |
| Explains values in plain words | Yes | Numbers only | Numbers only | Partly |
| Modern touch-friendly UI | Yes | Dated, desktop-first | Yes | Yes |
| Share as a link | Yes | Yes | Via account | No |
Falstad is still the best choice for advanced analogue and digital work with hundreds of parts, and Tinkercad wins for Arduino projects. Circuit Lab aims at the first steps: understanding why a circuit does what it does.
Click a part in the parts bin, then click on the board to place it (press R to rotate). Drag from any terminal dot to another to draw a wire — wires bend automatically and connect wherever they touch a terminal or another wire's end. Press play and the circuit comes alive.
It solves Kirchhoff's laws with Modified Nodal Analysis, the same method SPICE uses. LEDs, diodes and transistors use exponential (Shockley and Ebers–Moll) models solved with Newton–Raphson iteration, and capacitors and inductors are stepped through time. Batteries have a small internal resistance, so a short circuit gives a large but finite current, just like real life.
An LED has almost no resistance once it lights, so connected straight to a battery the current shoots far past its roughly 20 mA rating (most small LEDs list 20–30 mA as the absolute maximum). Add a resistor in series: R = (battery voltage − LED voltage) ÷ 0.015 A, for example about 470 Ω for a red LED on 9 V. Select the burnt LED and press 'replace' to try again.
Moving dots show conventional current flowing from + to −; the faster they move, the larger the current. Wire colour shows voltage: bright green is the most positive, grey is 0 V and red is negative.
Hover or click any part to see its voltage, current and power. For a physical-style measurement, place a voltmeter across a part (in parallel) or an ammeter in line with it (in series).
Your work is saved automatically in your browser, you can store named circuits, export a PNG image, and the share button copies a link that contains the whole circuit — anyone who opens it sees exactly what you built.
Tap a part then tap the board to place it, drag from a terminal to wire, pinch to zoom and drag the empty board to pan. Tap a switch to flip it and hold a push button to press it.
Students learning Ohm's law, series and parallel circuits, capacitors and transistors; teachers who want a quick demo on the projector; and makers who want to sanity-check an LED resistor or a transistor switch before soldering.
Values accept SI prefixes: 4k7, 10k, 2.2M, 100u, 47n.