From a clicking relay to a working 8-bit CPU.
DigiSim is two labs that share one simulator: flip real electromechanical relays in the Relay Lab, then wire logic gates all the way up to an 8-bit CPU in the Logic Lab. Narrated lessons in 7 languages, no install.
DigiSim Relay Lab
Build real digital logic from electromechanical relays. Flip A, B and the carry-in, watch the sum and carry-out compute live, and hear every contact click — right in your browser, no install.
DigiSim Logic Lab
The full digital-logic workbench. Build with 60 components — from the AND gate to RAM, ROM and an 8-bit ALU — follow narrated SimCast lessons, and design custom CPUs you can run your own assembly on.
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.
Sixty components. One complete CPU stack.
From the AND gate you learned in week one, to the program counter, instruction register, and 8-bit ALU that run your own assembly. Nothing simulated halfway.
CPU and memory components arrive on the Pro tier. See pricing →
A path that ends where real computers begin.
Six rungs from a single gate to running your own machine code. Built around the same syllabus as a university Computer Organization course.
Logic Gates Beginner
Boolean algebra you can poke at. Build the truth table by hand, then watch it light up in silicon.
Combinational Circuits Beginner
Wire gates together to do something useful — pick signals, decode addresses, compare numbers.
Sequential & Memory Intermediate
Logic that remembers. The clock arrives, hazards become real, and you meet your first state machine.
Arithmetic & ALU Intermediate
Carry-lookahead, two's complement, an ALU that knows the difference between ADD and AND.
Memory & Bus Advanced
RAM, ROM, an address bus, a data bus. The pieces start to look like the diagrams in your textbook.
A working 8-bit CPU Advanced
Program counter, instruction register, control unit, flags. Write assembly, load it, watch fetch-decode-execute.
Buy it once. Own your stack.
No subscriptions. No recurring fees. Pay once for full access.
Build and simulate digital logic — from a single gate to a working CPU.
- 16 components
- 21 interactive lessons
- Save & share your circuits
- Runs in your browser
- 60 components
- 69 interactive lessons
- Everything in Free
- One-time, yours forever
Build a computer from electromechanical relays — the CODE journey, hands-on.
- 18 of 25 Blueprints
- Hands-on relay sandbox
- Limited relay count
- Follow the CODE journey
- All 25 Blueprints
- Unlimited relays
- Export your machines
- Build a full adder & beyond
Academic discount: sign up with a .edu / .ac.uk email — discount applied automatically at checkout. Secure payment by Stripe.
Open the simulator. Build the first thing.
You don't need an account to start. You don't need to install anything. Just pick a door.