Circuit sandbox · free build
Build any circuit
Place batteries, a signal generator, resistors, potentiometers, sensors, LEDs, lamps, buzzers, motors, capacitors, inductors, transistors, MOSFETs and a 555 timer on the board, and wire them up. The circuit is solved the moment you change anything: every voltage, every current, and the warnings a real circuit would give you. Nothing here can catch fire, so experiment.
Board
How to build: choose a part, then click a point on the board to place it (it spans two squares; use “New parts” to switch direction). Choose Wire and click two points to connect them; keep clicking to continue the wire, press Esc to stop. Click a part to see its readings and change its value. Your circuit is saved in this browser automatically.
Build challenges
Each challenge checks how your circuit behaves, not how it is drawn, so any working design counts. They tick themselves as soon as your circuit meets the goal.
This is an example or a shared circuit exactly as it was opened, so it does not count towards the challenges. Change it or build your own.
Build with forms
Everything you can do on the board you can also do here, using the keyboard. Columns run 1–16 from left to right; rows run 1–10 from top to bottom.
Everything on the board
What the sandbox simulates
The board is solved with the same nodal-analysis solver as the measurement lab. Wires have 1 mΩ of resistance, and batteries have a small internal resistance (0.1 Ω unless you change it). LEDs and diodes are either off or hold a fixed forward voltage (0.7 V for the diode). Lamps are fixed resistors, although a real filament’s resistance rises as it heats up.
Capacitors are simulated through time in steps of up to 5 ms (backward Euler), so they charge and discharge with the time constant τ = R × C. Very fast circuits (τ much shorter than 5 ms) settle almost instantly here. Transistors use the model from the explainers: a base–emitter junction of about 0.7 V, collector current β × base current until the load limits it, then saturation at about 0.2 V. Real β varies a lot between parts. The MOSFET is a switch: off below its threshold voltage, otherwise a small resistance; it draws no steady gate current.
Voltages are measured from the battery’s negative terminal (or the signal generator’s), or from a Ground symbol if you place one.
New parts: the potentiometer is two resistors that share a wiper; the light sensor and thermistor use the same models as Circuit lab 02; the buzzer is an “active” buzzer (a fixed resistance that sounds above about 60% of its rated voltage); the motor and inductor are a resistance plus an inductance, without the back-EMF a real spinning motor makes. The signal generator has a 50 Ω output, like a bench generator. The 555 timer is a behavioural model: an internal divider sets the ⅓ and ⅔ thresholds, its output sits about 1.5 V below VCC when high (like the bipolar NE555), and its discharge pin shorts to ground when low.
After every change, the sandbox takes a few one-microsecond steps. If a coil’s current was just interrupted, that reveals the voltage spike a real circuit would produce.
A circuit that works here should work on a breadboard with the same values, but real parts have tolerances and limits. Build with batteries or USB power, and never short a battery: the sandbox shows the current, and a real battery would get hot.