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DigiSim Relay Lab

See what computing is really made of

Digital logic, built from the clicking relays that ran the first computers — and simulated for real, right here.

Open the Relay Lab $12.99 one-time

Free to start · no login · runs in your browser

Fig. 1 — This is a real simulated circuit — turn your sound on 🔊 and flip the switch. INTERACTIVE · TURN SOUND ON
CHAPTER 1 · THE CRESCENDO

Same relay, wired a little more cleverly

Everything a computer does is this one trick, repeated. Flip each circuit — every contact click is the real recorded sound.
  1. RUNG 01

    Two switches in series — that's AND

    Current only flows if A and B are both closed. Logic, before a single chip existed.

    Fig. 2 — Two switches in series — that's AND INTERACTIVE · FLIP A AND B
  2. RUNG 02 — THE CLIMAX

    Six relays carry the one

    A full adder: two bits plus a carry, computed by physics. Still nothing but relays.

    Fig. 3 — Six relays carry the one INTERACTIVE · THE CLIMAX
  3. RUNG 03

    Feed it back — it remembers

    Cross two relays and the circuit holds a bit after you let go. Memory is born.

    Fig. 4 — Feed it back — it remembers INTERACTIVE

  4. …and on to a computer

    Counter, clock, registers, CPU — the same climb continues in the Lab. The summit builds are part of the unlock.

    See what's in the unlock

    Following the arc of Charles Petzold's Code

CHAPTER 2 · WHAT YOU OPEN

What you open

The Relay Lab is a full editor: drag battery, switch, relay and lamp onto the canvas, wire them up, and a real DC solver computes every wire. Below — the actual product surface, live.

Live: the full adder, with the Lab's readout panel and zoom tools. INTERACTIVE · ZOOM & PAN
  • § 2.1

    A real DC circuit

    Every wire solved with Ohm's and Kirchhoff's laws — it computes because the physics works.

  • § 2.2

    Mechanically faithful

    Armatures pull in with authentically recorded relay clicks. Hear it think.

  • § 2.3

    25 Blueprints

    From one relay to a binary counter — every chapter of the story, pre-built and flippable.

  • § 2.4

    Runs in your browser

    No install, no login, no setup — open a Blueprint and start flipping.

Open the Relay Lab
CHAPTER 3 · OWNERSHIP

Start free. Unlock the summit.

The free Lab is genuinely useful — the unlock is one payment, not a subscription.
FREE — SIX RUNGS OF THE CLIMB THE UNLOCK — THE SUMMIT
Free $0
  • Circuits with up to 6 relays
  • 18 of the 25 Blueprints
  • All gates, the full adder, latches, the oscillator
  • No account needed
Open the Lab — free
THE UNLOCK
Relay Lab unlock $12.99 one-time
  • Unlimited relays
  • All 25 Blueprints — incl. the 8-bit adder and binary counter
  • Export your circuits as .relay files
  • One-time purchase — about the price of the Code paperback
Unlock in the Lab — $12.99
CHAPTER 4 · THE THEORY

Learn it the way it was invented

The Blueprints follow Charles Petzold's Code — the same arc from a telegraph relay to a working computer. We wrote an 18-part series that walks every rung, each with a live circuit inside.

Read the 18-part series Prefer guided lessons? Follow the SimCast course
FROM THE SERIES
REF. 01 How a Relay Works: The Click That Built Modern Computing REF. 02 Full Adder from Relays: Carry the One — and Add Any Number REF. 03 The RS Latch: How a Circuit Remembers — The Birth of Computer Memory
CHAPTER 5 · APPENDIX

Questions, answered

What do I get free, exactly?

Circuits with up to 6 relays and 18 of the 25 Blueprints — all gates, the full adder, latches and the oscillator — with no account. The $12.99 one-time unlock adds unlimited relays, all 25 Blueprints (8-bit adder, binary counter) and .relay export.

Do I need to install anything?

No. The Lab runs in any modern browser on desktop or mobile. Open it and flip a switch.

Is it really simulated, or just animated?

Really simulated: every wire is solved as a DC circuit (Ohm + Kirchhoff), relays actuate on mechanical timescales, and the clicks are recordings of real relays.

Refunds? Classroom use?

The unlock is covered by our standard refund policy. For classrooms, the free tier needs no accounts — every student can open the Lab instantly. Refund policy

Hear a computer think

Open the Relay Lab and flip your first gate to life.

Open the Relay Lab Explore DigiSim Logic Lab