Relay LabReleased

Logic you can hear click.

Explore visible coils and contacts, from relay logic to contactor control and on-delay/off-delay timers. A DC network model drives the mechanisms; recorded relay clicks accompany contact movement.

Open the Relay Lab Browse all Relay lessons

Free to start · no login · runs in your browser · turn your sound on

Relay LabLIVE BENCH

An interactive relay model with visible contacts and synchronized click audio.

Open the Relay Lab

112cataloged Blueprints

37 curated starters plus 75 archived lesson-staging circuits in the captured production catalog.

102published course items

89 instructional lessons and 13 checkpoints, ordered from closed circuits to arithmetic and memory.

DC modelfor nodes and branches

A resistive nodal solve calculates voltages, coil currents, supply current, and lamp brightness for the modeled circuit.

18free · no account

Direct answer

What is DigiSim Relay Lab?

DigiSim Relay Lab is an online simulator of electromechanical control and logic. Connect batteries, manual switches, buttons, relays, contactors, timers and lamps. A resistive DC network model calculates coil current, while the actuation model moves linked contacts and times TON/TOFF transitions in simulation time. It is an educational DC model, not an industrial equipment certification tool.

Best for: learners studying how relay contacts implement logic, readers exploring the conceptual progression in Charles Petzold’s Code, introductory computer-architecture courses, and technical demonstrations of audible relay switching.

RelayCastRelay Lab

102 published course items

Guided Relay course

From a click to a computer, one rung at a time.

The Relay course follows a path from switches and relays through gates, arithmetic, feedback, and memory. Its published catalog contains instructional lessons and checkpoints; 81 items include archived interactive circuit stagings, while 21 are worksheet-style items.

The first 18 course items, beginning with the closed loop and relay control, are public. Pro opens the complete published course and Blueprint catalog.

Browse all Relay lessons

L06 · M0A circuit is a loop

L07 · M0The switch

L08 · M0Series and parallel

L09 · M0The short circuit

L10 · M0One common return

L12 · M1A switch worked by electricity

L13 · M1NO, NC and the change-over

L14 · M1The relay as amplifier

MCC · Codes and combinations5
M00 · Electricity you can see6
M11 · The relay7
M22 · Logic with switches8
M33 · Gates9
M44 · Building blocks10
M55 · Binary and arithmetic9
M66 · Feedback and latches7
M77 · Clocks and flip-flops7
M88 · Memory7
M99 · State machines7
M1010 · Industrial control13
M1111 · Toward a computer7

From one switch to a working adder in four steps

How it works

You wire the modeled loop, close the switch, and inspect the electrical and contact states calculated by the simulator.

01
Close a loop
Battery, switch, lamp. Close the switch and the lamp lights because the circuit is complete, not because a rule says so.
02
Let a coil do the switching
Put a relay coil in one circuit and use its contacts in another. When calculated coil current crosses the pickup threshold, the actuation model changes the contact state after its operate delay.
03
Wire contacts in series and parallel
Under a defined active-high convention, normally open contacts in series can implement AND and contacts in parallel can implement OR. A normally closed contact can provide inversion in an appropriate relay circuit.
04
Build up, then listen
Gates become a half adder, a full adder, a latch, a counter. Turn the sound on: a computer made of relays has a rhythm.

Start a new bench

Relay logic from switching fundamentals to arithmetic and memory

A bench, not an animation

The Relay Lab pairs a focused palette with a DC network model. Calculated coil current crosses explicit pickup or dropout thresholds, and the actuation layer then changes the modeled contacts after a defined delay.

A resistive DC network model

The solver represents batteries, wires, closed switches, relay coils, contacts, and lamps as a nodal DC network. Series and parallel contacts can implement Boolean functions when the surrounding circuit and signal convention are defined.

Modeled electromechanical timing

Separate operate and release delays make contact transitions visible over time. Recorded pickup and release samples provide the primary audio, with synthesized clicks as a fallback when samples are unavailable.

112 cataloged Blueprints

From a single relay to a full adder, an RS latch, an oscillator and a binary counter. Open one, flip it, then rewire it.

A progression familiar from Code

The catalog follows a conceptual path from switches and relays through gates, arithmetic, feedback, and memory. This is an independent learning resource, not a chapter-by-chapter edition of the book.

Shareable, embeddable

Share a supported relay circuit through a revocable public link, or use the Relay interactive viewer where embedding is supported.

Runs in your browser

Open the bench without an account. Try every component, with up to six relays, contactors and timers combined per circuit. Each device counts once, regardless of its contacts.

Who it is for

For the moment a gate stops being a symbol

Use it where a real relay bench is impractical, where a class should hear the same machine at once, or where the question is not what a gate does but why.

Reading Code

Use selected Blueprints alongside the book’s progression from switching and relays to logic and memory. Coverage is conceptual rather than a one-to-one chapter mapping.

First architecture lessons

Begin below the gate-symbol abstraction and let students observe how contact networks produce sum and carry behavior in a relay adder.

Museums and demonstrations

Project a live full adder, let a visitor flip the inputs, and turn the sound up. The embed on this page is the same viewer you can put in yours.

Try every component. Build larger control circuits with Pro.

Honest access

Guests and Free accounts can use every component, with six control devices per circuit. Relays, contactors, TON and TOFF timers each count once; batteries, manual inputs, lamps, wires and annotations do not count. Simulation and timing controls work the same on every plan.

Free

Build and keep a small control circuit

Start without an account, keep one browser-local draft, and download your own .relay project within the free circuit limit. Sign in to save up to five circuits in the cloud and create supported public shares.

18 / 102RelayCast lessons free

Battery, switches, coils, contacts and lamps
Six relays, contactors and timers combined per circuit
Revocable public links for your own circuits
No account needed

Open the free bench

ProINCLUDED WITH PRO

Build larger systems and explore the full course

Remove the Free-tier control-device cap and explore larger circuits and the complete published Relay course. Pro includes up to 250 cloud circuits and supported image exports. Technical circuit and sharing limits still apply.

112Blueprints, without the Free-tier device cap

No Free-tier control-device cap
All 112 Blueprints and all 102 published course items
Export PNG and SVG images of your circuits
Cloud save and sharing

See Pro plans

Questions

Before you open it

Is it really simulated, or just animated?

It uses a circuit model rather than a prerecorded animation. A nodal DC solver calculates the modeled network, calculated coil current drives pickup and dropout thresholds, and a separate timing layer schedules contact movement. Recorded relay samples are used when available, with synthesized audio as a fallback.

What do I get free, exactly?

Every component is available to guests and Free accounts. Each circuit can contain six relays, contactors and timers combined. Each counts once; manual inputs, batteries, lamps, wires and annotations do not count. Local .relay downloads are free within that allowance. Signing in adds cloud saves and public sharing; Pro and Founder remove the Free-tier device cap and include the complete published Relay course and image exports.

Do I need to install anything?

No. Use a current desktop version of Chrome, Firefox, Safari or Edge. Small touch screens are better for viewing than for wiring.

Can I put a relay circuit on my own page?

Yes. Relay Blueprints have an interactive embed viewer; the one at the top of this page is it. Public links are revocable at any time.

Does it follow a book?

The catalog and course use a related conceptual progression from switching and relays through logic, arithmetic, and memory. They are independent resources rather than a chapter-by-chapter adaptation, and the book is not required.

Hear a computer think.

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

Open the Relay LabFree to start · no login · runs in your browser