PLATE A — 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
Explore the Relay Lab Free to start · $12.99 one-time unlock
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.
No account · no install · saves locally
60 components
25 Blueprints
66 narrated lessons
7 languages
18 -part written series
CHAPTER 1 · THE CLIMB
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.
Every step is a real, probeable simulation — the page you're on is running them.
CHAPTER 2 · Pick your door
Two labs, one workshop. Start anywhere — they tell one story.
PLATE B — 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
Explore the Logic Lab Free to start · Pro $32 one-time
CHAPTER 3 · 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.
EDITIONS — English · 中文 · 日本語 · Español · 한국어 · Deutsch · Français Browse the lesson catalog
CHAPTER 5
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.
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REF. 01How a Relay Works: The Click That Built Modern Computing
How does a relay actually work? Explore the electromechanical switch behind every computer — from the telegraph to the logic gate — and toggle a live relay right in your browser.
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REF. 02How an ALU Works: Arithmetic Logic Unit from Gates
An Arithmetic Logic Unit performs every math and bitwise operation a CPU executes. See how a 4-bit ALU is built from adders, comparators, and a multiplexer.
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REF. 03Build a CPU from Scratch in a Simulator
A complete walkthrough for building a working 8-bit CPU from logic gates: registers, ALU, bus, RAM, control unit, and a runnable three-instruction program.
CHAPTER 6
Buy it once. Own your stack.
No subscriptions. No recurring fees. Pay once for full access.
LOGIC LAB
Free
RECOMMENDED
LOGIC LAB
Pro
RELAY LAB
Free
RELAY LAB
Full
PRICE
$0
$32 ONE-TIME
$0
$12.99 ONE-TIME
COMPONENTS
16
All 60
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—
NARRATED LESSONS
22
All 66
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—
RELAY BLUEPRINTS
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—
18 of 25
All 25
RELAY COUNT
—
—
Up to 6
Unlimited
Table 1 — Editions & pricing. Purchases are one-time; the labs are separate products. SECURE PAYMENT BY STRIPE
Academic discount: sign up with a .edu / .ac.uk email — discount applied automatically at checkout. Secure payment by Stripe.
Start at the click. End at a computer.
No account. No install. The first relay is one tap away.
free · no account · in your browser Start building — free