Open a routed board

circsim's one required input is a routed .kicad_pcb file (KiCad 6 to 10). There are three ways to open one.

From the start screen

With no board loaded, the viewport offers:

• Open sample project: the bundled 555 blinker.
• Open First Light demo: the bundled one-LED dimmer.
• Open…: a file picker for your own board.
• Recent boards: the boards you opened before, newest first (see Save your setup & export a report).

A third bundled sample, sensor-node.kicad_pcb (94 parts: ESP32 module, regulator, shift registers, 16 LEDs, sensors, switched loads), has no button on the start screen. It is in the sample folder next to the other two (in the repository: resources/sample/); open it with Open… or drag it onto the window. circsim puts the bench supply on the 3.3 V rail by default, so attach it to +5V instead, which is the board's input.

From the header

The Open… button in the top bar opens a file picker at any time. Opening a new board replaces the current one.

If the board does not load cleanly

• A KiCad 5 or older file is refused with a message to save it from KiCad 6 to 10 first.
• A missing or broken board outline does not stop the board from opening. The 3D view then shows no substrate or a bounding-box substrate, and a warning row above the viewport says why. See supported files.

By drag and drop

Drag a .kicad_pcb from your file manager straight onto the circsim window. circsim reads only files you open this way, with the Open button, from the recent list, or beside an opened board (the schematic and BOM), never other paths on your disk. (You can also drop a .kicad_sch this way to attach a schematic to an already-open board.)

What happens next

The moment a board loads, circsim:

1. renders it in 3D (drag to orbit, scroll to zoom);
2. rebuilds the circuit from the copper and fills the Parts panel; each part gets a colored dot for its model status;
3. auto-suggests a ground net and supply nets in the Ground & Power panel;
4. restores the setup saved beside the board (my-board.circsim.json: ground, bench, overrides, models), if there is one, replacing those suggestions, and tells you what it restored;
5. runs the static Board Critic checks (floating nets, clearance, decoupling, loop area).

Parsing, netlist extraction, model resolution and the Board Critic run in a background worker, so the window stays responsive while a large board opens. A strip under the toolbar names the stage (parsing, extracting, resolving, then running the Board Critic). The board appears as soon as it is resolved; the Board Critic panel fills in when its checks finish, which on a very large board (thousands of parts, tens of thousands of tracks) takes a second or two longer than the board itself. If you open another board in the meantime, the earlier open is abandoned.

The header shows a live summary, "12 parts · 10 ok · 2 unresolved", so you immediately see whether anything needs attention.

It parsed but something's off?

If circsim can't parse the file it shows a specific error (with a line/column). If it parses but a part is unresolved, that's normal: head to the Model Doctor. If the netlist is unusable (no parts or nets), circsim shows the board read-only with a viewer-only badge.

Next

• Attach a schematic for higher-fidelity models and correct diode polarity.
• Set ground & supply, then Energize.