Subcircuits in Logisim-evolution

Every Logisim-evolution project is a library of circuits. A subcircuit is a smaller circuit used as a module inside a larger one — the hardware equivalent of a function in programming. Once your designs grow past a handful of gates, subcircuits are not optional.

Creating a subcircuit

Select Project → Add Circuit… and type a name (it must start with a letter). Build your circuit there using Input Pins and Output Pins for its interface. Back in the main circuit, the new circuit appears in the explorer pane — select it and click to place it like any gate.

Editing the appearance

Select the subcircuit in the explorer and click the Appearance tool (the paintbrush icon in the toolbar). You can rearrange the pin positions, add labels, and draw a custom outline so the block reads clearly in the parent circuit. This is the step most beginners skip — and then cannot tell their own blocks apart.

Reusing and sharing

A subcircuit can be placed as many times as you like; each instance is independent. To reuse circuits across projects, use Project → Load Library → Logisim-evolution Library… and pick the .circ file. Our adders tutorial uses this exact pattern: build the half adder once, instantiate it twice inside the full adder.

Debugging inside subcircuits

To watch a subcircuit while the top circuit runs, open Project → View Simulation Tree and double-click the instance. A red wire inside a subcircuit usually means a floating pin — see troubleshooting.