Circuit Sandbox
Lay wires, connect components, and explore complete circuits.
Keyboard building
W: wire, B: source, L: bulb, T: transformer, S: switch, M: move/inspect, E: erase, H: pan, R: rotate. Arrow keys move the grid crosshair; Enter places a component or adds a wire bend. Enter twice at the same spot finishes a loose wire. Escape cancels a wire. Backspace removes its last bend. In Move / inspect, Enter selects a component; its properties appear below. Select a component, move the crosshair, then press Shift+Enter to move it. Delete removes the selected component. Switches toggle from their inspector or with Space when selected.
How to play
- Place parts: choose a battery, lightbulb, resistor, transformer, or switch and click empty grid space. Rotate changes the placement direction. Move / inspect lets you select, edit, or drag an existing component.
- Lay wires: drag a path across the grid, or click a starting contact and add bends with clicks. Finish on a contact or an existing wire. Enter finishes a loose end; Escape cancels. Every crossing and shared segment is an electrical connection, like roads in Traffic Jam.
- Close the loop: connect a source through a bulb and back to its other terminal. A bulb glows according to its power. Open a switch or erase a wire to break the loop. The source readings update after every edit.
- Try transformers: use the Transformer example or switch to AC generators. The two left contacts are the primary; the two right contacts are the isolated secondary. Connect a return path on each side. Edit the turns ratio to step voltage up or down.
- Expand: use Pan, mouse-wheel zoom, or the + / − buttons to explore the large board. Fit circuit brings your layout into view. Undo restores edits.
Electrical model and readings
Bulbs and resistors use fixed resistance and Ohm’s law. Bulb glow follows I²R relative to 5 W; this does not model a filament’s changing temperature or burnout. Wires are ideal conductors; connected crossings share one voltage. Switches have 0.02 Ω when closed and are open circuits otherwise. Each source has 0.2 Ω internal resistance, so terminal voltage drops under load and shorts have a finite calculated current.
The solver uses Kirchhoff’s laws for the complete network, including branching loops and multiple sources. Source output current is signed: negative means that source absorbs power from another. External equivalent resistance is measured at the selected source’s terminals with that source removed and all other independent sources set to zero. It excludes the selected source’s own internal resistance. “Open” means no finite load path. A closed network with almost no input current can still be unloaded.
AC mode replaces all batteries with synchronized, in-phase sinusoidal generators. AC readings are RMS. Transformers use an ideal turns ratio with 0.2 Ω resistance in each winding; secondary load is reflected back to the primary, so voltage, current, and winding losses are calculated together. A transformer does not transfer steady DC power: in DC mode the windings act as separate 0.2 Ω resistors. A battery can therefore drive high current through its primary while the secondary bulb remains dark. The model omits magnetic saturation, magnetizing current, frequency response, and switching transients.
Wire colors use local voltage references: disconnected or transformer-isolated networks have independent references, so their absolute wire colors cannot measure voltage between isolated circuits. The solver uses tiny stabilizing conductances for floating nodes; currents below 0.1 µA are displayed as zero. This educational model is not a simulator for building mains-voltage equipment.
Up to 40 components and 300 wire segments. Place components on empty grid space, then connect their contact circles. When moved or rotated, wire vertices terminating at a contact follow that contact; connections touching the middle of a wire should be redrawn. Layouts stay in this page while open and are not saved to an account.
References: OpenStax — Kirchhoff’s Rules and Transformers.