Key Takeaways

  • CircuitJS is a completely free, open-source circuit simulator that runs in any modern web browser — no installation required.
  • Real-time animated current flow helps beginners visualize exactly how electricity moves through resistors, capacitors, LEDs, and transistors.
  • Built-in tools like an oscilloscope and voltmeter let you measure voltage, current, and frequency just like real lab equipment.
  • You can simulate Arduino-compatible circuits — including voltage dividers, LED arrays, and motor drivers — before touching a single physical component.
  • Circuits can be shared instantly via URL, giving you access to thousands of community-built examples to learn from and remix.
  • Combining CircuitJS simulation with physical breadboarding reduces component waste and dramatically accelerates your learning curve.

What Is CircuitJS and Why Should You Use It?

What if you could test any circuit before soldering a single wire — for free, right in your browser? CircuitJS makes that possible, and once you try it, you'll never breadboard blind again. Originally developed by Paul Falstad and later ported to JavaScript by Iain Sharp, CircuitJS (also known as Falstad Circuit Simulator) is a fully open-source, browser-based electronics simulation tool that has become a staple for hobbyists, students, and educators worldwide.

Unlike heavyweight simulation suites such as LTspice or Multisim, CircuitJS requires zero installation and zero cost. You simply navigate to falstad.com/circuit/circuitjs.html and start building immediately. The simulator handles analog and digital components, reactive elements like capacitors and inductors, and even logic gates — all rendered in real time with animated current flow that makes abstract circuit theory suddenly click.

For Arduino beginners and Raspberry Pi enthusiasts, this CircuitJS tutorial will show you how to validate your circuit ideas before committing components to a breadboard. This single habit alone can save you hours of debugging and prevent the frustrating experience of burning out an LED or a microcontroller pin with an incorrectly calculated resistor.

Getting Started: The CircuitJS Interface Explained

When you first open CircuitJS, you're greeted with a dark canvas populated by a sample circuit — typically a simple RLC or oscillator circuit to demonstrate the simulator's real-time animation. The interface is intentionally minimal. The main canvas takes up most of the screen, with a menu bar along the top providing access to components, drawing tools, and simulation controls.

Navigating the Canvas

The canvas behaves like an infinite grid. You can zoom in and out using your mouse scroll wheel and pan by clicking and dragging on an empty area. Right-clicking on any component opens a context menu that lets you edit its properties, delete it, or flip its orientation. This non-destructive editing approach means you can experiment freely without fear of breaking anything permanently.

The Draw menu at the top is where you'll spend most of your time. It's organized into logical categories: Passive Components (resistors, capacitors, inductors), Inputs and Sources (voltage sources, current sources, clock signals), Outputs (LEDs, light bulbs, logic probes), Active Components (transistors, op-amps, diodes), and Logic Gates. Each category expands into a submenu, giving you access to well over 100 distinct component types.

Placing Your First Component

Placing a component in this CircuitJS tutorial is as straightforward as selecting it from the Draw menu and then clicking and dragging on the canvas to define its orientation and size. For a resistor, for example, select Draw → Passive Components → Resistor, then click where you want one terminal to start and drag to where you want the other terminal to end. Release the mouse and the resistor snaps to the grid.

Wires work the same way — select Draw → Wire (or press W as a shortcut) and connect component terminals together. CircuitJS automatically detects junctions and highlights connection nodes in green when components are properly linked. If a wire is floating or a node is disconnected, the simulator will warn you when you try to run the simulation, making it easy to catch wiring errors early.

Real-Time Current Flow Visualization

One of the most powerful features highlighted in any CircuitJS tutorial is the real-time current flow animation. As soon as your circuit is complete and the simulation is running, you'll see small animated dots flowing along the wires. These dots represent conventional current direction, and their speed and density correspond to the magnitude of the current at that point in the circuit.

Color coding adds another layer of information. Wires carrying positive voltage are shown in a warm color (typically yellow or orange), while negative or ground references appear in cooler tones. This visual feedback is extraordinarily useful for debugging: if you expect current to flow through a branch but the dots stop at a particular component, you've immediately identified your fault. For beginners learning Ohm's Law or Kirchhoff's Current Law, watching the animated dots split at a parallel junction and recombine afterward is worth more than a dozen textbook diagrams.

Adjusting Simulation Speed

The Simulation Speed slider in the right-hand panel controls how fast the simulation runs relative to real time. For high-frequency circuits like oscillators or PWM signals, you'll want to slow the simulation down to observe individual cycles clearly. For slow RC charging circuits, speeding it up saves you from watching paint dry. This flexibility makes CircuitJS equally effective for studying 60 Hz AC mains behavior and 10 MHz digital clock signals.

Building an Arduino-Compatible Circuit: Voltage Divider Example

A voltage divider is one of the most common circuits in Arduino projects — used to scale a 5V signal down to 3.3V for a sensor input, for example. Building one in CircuitJS before wiring it physically is an excellent way to verify your resistor values and confirm the output voltage before risking your microcontroller's analog pin.

To build a voltage divider in CircuitJS, start by placing a DC voltage source set to 5V. Then place two resistors in series between the positive terminal and ground. For a classic 10kΩ / 10kΩ divider, you'll see the midpoint node settle at exactly 2.5V as soon as the simulation runs. Right-click either resistor and select Edit to change its resistance value and watch the output voltage update in real time — no recalculation needed.

Simulating an LED Circuit

LEDs are another staple of Arduino projects, and incorrectly sized current-limiting resistors are one of the most common beginner mistakes. In CircuitJS, place a voltage source (5V), a resistor, and an LED in series. The simulator will show you the current flowing through the LED in real time. A standard red LED has a forward voltage of approximately 2.0V and a safe operating current of 20mA — use the formula R = (Vcc - Vf) / If to calculate your resistor, then verify it in the simulator before purchasing components.

CircuitJS also lets you right-click the LED and view its instantaneous voltage drop and current, which is equivalent to probing it with a multimeter. This kind of instant feedback reinforces the math behind component selection in a way that passive reading simply cannot replicate.

Motor Driver and Transistor Circuits

For more advanced Arduino projects involving DC motors, CircuitJS supports NPN and PNP BJT transistors as well as MOSFETs. You can simulate a classic NPN transistor switch — where a small base current from an Arduino digital pin controls a larger collector current driving a motor — and measure the saturation voltage and power dissipation before selecting a physical transistor. This is particularly valuable when deciding between a 2N2222 for low-current loads and a TIP120 Darlington transistor for heavier motor loads.

Using the Built-In Oscilloscope and Measurement Tools

A professional oscilloscope can cost hundreds of dollars, but CircuitJS includes a fully functional virtual oscilloscope that you can attach to any node in your circuit for free. This is one of the features that elevates CircuitJS from a simple sketch tool to a genuinely educational simulation environment.

To open the oscilloscope, right-click any wire or component terminal and select View in Scope. A scope window will appear at the bottom of the screen, plotting voltage (or current) over time for that node. You can add multiple traces to compare signals — for example, comparing the input and output of an RC low-pass filter to visually observe phase shift and attenuation. The scope automatically scales its axes to fit the signal, though you can manually adjust the time/division and voltage/division settings for precise measurements.

Measuring with the Voltmeter and Ammeter

In addition to the oscilloscope, CircuitJS provides dedicated voltmeter and ammeter components found under Draw → Outputs. Place a voltmeter across any two nodes to see a continuous DC or AC voltage reading. Place an ammeter in series with any branch to measure current flow. These tools behave exactly like their physical counterparts, reinforcing good measurement habits — such as always connecting a voltmeter in parallel and an ammeter in series — that will serve you well when you move to real test equipment.

Sharing and Learning from Community Circuits

One of CircuitJS's most underrated features is its URL-based sharing system. Every circuit you build can be exported as a compact URL that encodes the entire schematic. Share that URL with a friend, post it in a forum, or bookmark it for later — when anyone opens the link, they see your exact circuit running in real time, ready to interact with and modify.

The official CircuitJS repository on GitHub contains the full source code and links to community contributions. Beyond the official repo, sites like Reddit's r/electronics and Arduino forums regularly share CircuitJS links when helping beginners debug circuits. This makes CircuitJS an ideal collaborative learning tool — instead of describing a circuit in words, you can share a live, interactive simulation in seconds.

Loading Built-In Example Circuits

If you're new to electronics, the fastest way to get up to speed with this CircuitJS tutorial is to explore the built-in example circuits. Under Circuits in the menu bar, you'll find dozens of pre-built examples organized by category: Basics, Filters, Amplifiers, Oscillators, Digital Logic, and more. Loading an example and then modifying component values is one of the most effective learning strategies available — you immediately see how changing a capacitor value shifts a filter's cutoff frequency, or how increasing base resistance affects transistor gain.

Tips for Getting the Most Out of CircuitJS

After working through this CircuitJS tutorial, a few practical habits will help you simulate more effectively. First, always label your nodes using the text tool — this makes it much easier to identify signals when you have multiple oscilloscope traces open. Second, use the Stop button to pause the simulation before making significant changes to your circuit; editing a live simulation can sometimes cause voltage spikes that confuse the solver.

Third, pay attention to the Time Step setting under Options → Other Options. The default time step works well for most audio-frequency and DC circuits, but high-frequency RF or fast-switching digital circuits may require a smaller time step for accurate results — at the cost of slower simulation speed. Finally, remember that CircuitJS uses ideal component models; a simulated 1kΩ resistor has no tolerance, no temperature coefficient, and no parasitic inductance. Real-world circuits will always behave slightly differently, which is why combining simulation with physical breadboarding remains essential.

Integrating CircuitJS Into Your Breadboarding Workflow

The most effective workflow for Arduino and electronics hobbyists is a three-step process: simulate in CircuitJS, prototype on a breadboard, then finalize on a PCB or perfboard. The simulation step catches logical errors — wrong resistor values, incorrect transistor biasing, missing bypass capacitors — before you ever reach for a component. The breadboard step catches physical realities that the ideal simulator misses, such as wire resistance, LED color variation, and power supply noise. By the time you commit to a permanent build, you've already solved the vast majority of potential problems.

This workflow is especially valuable when ordering components online, where shipping times can be days or weeks. Simulating your circuit first means you order exactly the right components the first time, rather than discovering mid-build that you need a different transistor or a larger capacitor.

Start Building Today

CircuitJS is one of the most accessible, powerful, and genuinely useful free tools available to electronics hobbyists in 2026. Whether you're an Arduino beginner trying to understand why your LED keeps burning out, or an experienced maker validating a new motor driver design, this CircuitJS tutorial has shown you that the simulator covers everything from basic Ohm's Law demonstrations to complex multi-stage amplifier analysis — all without spending a cent or installing a single application.

The best way to learn electronics is by doing, and CircuitJS removes every barrier between you and that hands-on experience. Open the simulator at falstad.com/circuit/circuitjs.html, load one of the built-in examples, and start changing values to see what happens. Then come back to BreadboardHub for more tutorials, component guides, and project walkthroughs that take your simulated circuits all the way to finished builds. The canvas is blank, the components are free, and your next great circuit is waiting — start building today.