DigiSim — two labs, one workshop

From a clicking relay to a working 8-bit CPU.

Start at the atom of computing — one electromechanical relay — and climb to a CPU you program yourself. Real simulators, in your browser, free to start.

Start building — free Take the climb

No account · no install · saves locally

This relay is live — turn your sound on 🔊 and flip the switch.

How a click becomes a computer

Five steps from a clicking contact to running your own machine code. Three of them are real circuits, right on this page.

THE CLIMB 01
Wire two relays in series — you've invented AND

Current flows only when both coils pull. Every decision a computer makes is gates like this one, and this one is live.
THE CLIMB 02
Six relays carry the one

A full adder: two bits plus a carry, computed by physics. We built it relay by relay on the Relay Lab page.
Watch the adder come together
THE CLIMB 03
Feed it back — it remembers

Loop the output into the coil and the circuit holds a bit after you let go. This is what RAM is, multiplied by billions.
THE CLIMB 04
Trade iron for ideal switches

Same logic, drawn as symbols, driven by a clock — now it counts by itself. This is where the Logic Lab begins.
THE CLIMB 05
Assemble a computer

Registers, RAM, a program counter, an ALU. Wire them together, write your own assembly, and watch fetch–decode–execute.

Open the Logic Lab See the capstone build

Every step is a real, probeable simulation — the page you're on is running them.

Pick your door

Two labs, one workshop. Start anywhere — they tell one story.

DigiSim Relay Lab

Start from the physics

A real DC solver — Ohm and Kirchhoff on every wire
Authentic recorded relay clicks
25 Blueprints, from one relay to a binary counter

Free to start · $12.99 one-time unlock

Explore the Relay Lab

DigiSim Logic Lab

Build to the summit

60 components — gates to ALU, RAM and ROM
SimCast narrated lessons and ready-made templates
Design CPUs that run your own assembly

Free to start · Pro $32 one-time

Explore the Logic Lab or browse the template library

SimCast · Professor Mode

Lessons that don't just play. They teach.

Press play and a finished circuit comes alive. The lesson highlights one component, narrates the idea, then pans the camera to the next. Pause anywhere — every signal is still probeable, because it's still a simulator.

English中文日本語Español한국어DeutschFrançais

66 narrated lessons in the catalog Browse the lesson catalog

Put a live circuit anywhere

Every template and Blueprint embeds as a real, interactive simulator — in your course page, slides or blog. Not a screenshot: your students flip the switches.

embed.html

<iframe
  src="https://digisim.io/circuits/embed/template/and-gate-security-system?theme=dark"
  width="100%" height="420" loading="lazy"
  title="Interactive logic circuit" allow="autoplay"></iframe>

Live simulation, not an image
Works anywhere an iframe works
Free circuits stay free for your readers

Teaching with it? Academic emails get an automatic discount at checkout.

Browse circuits to embed

From the workshop journal

We wrote the climb down — an 18-part series following Charles Petzold's Code from one relay to a working computer, every post with a live circuit inside.

60components

25Blueprints

66narrated lessons

7languages