~/circuit-lab periodic table how it works ☕ Support me all apps ← home

Circuit Lab — free online circuit simulator for beginners

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.

Lesson 1 / 8

🎯

Build your first circuit ⚡

Nothing to install, nothing to sign up for.

1. Click a part on the left, then click the board to place it
2. Drag from a ● dot to another dot to draw a wire
3. Close the loop from + back to − — it runs instantly

# How to use the circuit simulator

Place parts: click any part in the bin (or drag it) and click the board. Press R to rotate while placing. Wire: drag from a terminal dot to another dot — the wire bends at right angles and snaps to the grid. Wires connect wherever they touch a terminal or another wire’s end, and a dot shows every junction. Interact: click switches, hold push buttons, and select a part to change its value with the slider, a preset, or by typing values such as 4k7, 10k or 100u.

Moving dots show conventional current (from + to −) and move faster when more current flows. Wire colour is voltage: green is positive, grey is 0 V, red is negative. Hover any part for its voltage, current and power — or open the scope to plot values over time.

# What makes it different

Explains, not just calculates

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.

Things break, like real life

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.

Lessons that check your work

Eight bite-size challenges, from lighting an LED to fixing a two-transistor blinker, with hints and automatic goal detection.

Real circuit maths

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.

Friendly on every screen

Tap to place, drag to wire, pinch to zoom. Works on phones, tablets, Chromebooks and projectors.

Private and free

No account, no ads, no upload. Circuits save in your browser and share as a link that contains the whole circuit.

# How it compares

Circuit LabFalstad CircuitJSTinkercad CircuitsPhET DC Kit
Signup neededNoNoYes (Autodesk)No
Animated current flowYesYesNoYes
Transistors, capacitors, ACYesYes, hundreds of partsYes + ArduinoNo (DC kit)
Guided lessons with goal checks8 built inNoSeparate lesson plansTeacher activities
Explains values in plain wordsYesNumbers onlyNumbers onlyPartly
Modern touch-friendly UIYesDated, desktop-firstYesYes
Share as a linkYesYesVia accountNo

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.

# FAQ

Is Circuit Lab free?
Yes. Every component, lesson, example and export is free, with no account, no ads, no watermark and no limit on circuit size.
How do I build a circuit?
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.
Is the simulation physically accurate?
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.
Why did my LED burn out?
An LED has almost no resistance once it lights, so connected straight to a battery the current shoots far past its ~20–30 mA limit. 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.
What do the colours and moving dots mean?
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.
How do I measure voltage and current?
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).
Can I share or save my circuit?
Yes. 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.
Does it work on a phone or tablet?
Yes. 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.
Who is it for?
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.

# Related tools

Periodic table Equation solver Study schedule generator 8-bit studio All apps