Digital Logic Foundations

Logisim Basics

Logisim Evolution is a free, cross-platform digital logic simulator used to design, build, and test gate circuits visually. This lesson covers the interface, components, and workflow.

What is Logisim?

Logisim Evolution is an open-source graphical logic simulator. It lets you drag-and-drop gates, connect wires, and simulate digital circuits in real time without any physical hardware.

Download: Logisim Evolution v2.14.6+ from https://github.com/reds-heig/logisim-evolution
Requires Java (OpenJDK ≥ 9). Runs on Windows, Mac, and Linux.

Interface Overview

Toolbar

Select, wire, and operate tools. The finger icon switches to simulation mode.

Explorer Pane

Browse gates, inputs, outputs, and custom components organised in categories.

Canvas

Main drawing area. Click to place components, drag to connect wires.

Attribute Table

Shows and edits properties of the selected component (e.g. number of inputs).

Step-by-Step: Build an AND Gate Circuit

Step 1 — Place Inputs

From the Explorer Pane → Wiring → Pin. Click twice on the canvas to place two input pins.

Step 2 — Place AND Gate

Explorer Pane → Gates → AND Gate. Click on the canvas to place it to the right of the pins.

Step 3 — Place Output

Explorer Pane → Wiring → Pin (set to "Output" in Attribute Table). Place it to the right of the AND gate. The red dot is the LED output.

Step 4 — Connect Wires

Hover over a pin endpoint — cursor changes to crosshair. Click and drag to the gate input. Repeat for all connections.

Step 5 — Simulate

Click the finger (operate) icon in the toolbar. Click input pins to toggle them. Observe the LED output changing.

Important Logisim Controls

Control	Function
Select tool (arrow)	Select and move components
Edit tool (pencil)	Draw wires
Operate tool (finger)	Toggle input pins during simulation
Attribute Table	Change gate fan-in (number of inputs), etc.
File → New	Create a fresh canvas (keep previous in another tab)
File → Save	Save circuit as .circ file

Changing Gate Input Count

By default AND/OR gates have 5 inputs. Change to 2 via the Attribute Table:

Select the gate on the canvas
Look at the Attribute Table (bottom-left panel)
Find "Number of Inputs" (4th row)
Click and change the value to 2

Power Supply: In Logisim, the power supply is universal and invisible — you don't need to wire Vcc or GND. In real circuits, every chip needs +ve (Vcc) and -ve (GND) rail connections.

Inputs and Outputs in Logisim

Inputs can be:

Binary switches — toggle manually in operate mode
Buttons — pulse high while clicked
Clock generators — generate automatic square waves

Outputs are typically:

LEDs (lights) — red when high, grey when low
7-segment displays — for multi-bit output

Lab Tip: Always verify your circuit by filling in a truth table manually first. Then test all input combinations in Logisim and check that the LED output matches your table.