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Logic Gates from Relays
Capvideo (Wikimedia Commons) · CC BY 4.0
Ed

Nilikha ni

Ed

27. Agosto 2026FI
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Logic Gates from Relays

A relay is a switch operated by an electromagnet, invented to repeat telegraph signals over long lines and already in this catalogue for that purpose. It has one property its inventors had no reason to care about: because a relay is a switch that ELECTRICITY operates, the output of one relay can operate the next. That is the whole of digital logic. Claude Shannon noticed in 1937, in a master’s thesis usually called the most important of the century, that a network of such switches behaves exactly like Boolean algebra — series contacts are AND, parallel contacts are OR, and a normally-closed contact is NOT. Suddenly a body of mathematics from 1854 became a design method for circuits, and circuit design stopped being intuition and became algebra you could simplify on paper before touching a soldering iron.
Katamtaman
5 hours 30 minutes

Mga Tagubilin

1

Start from the telegraph relay

Build or re-read the relay first. It was invented to solve a completely different problem — a telegraph signal too weak to travel further, restored by using it to operate a local switch on a fresh battery.

Note the property that matters here and mattered not at all to its inventors: the thing that operates the switch is the same KIND of thing the switch controls. A lever needs a hand; a relay needs only another relay. That closure under composition is what makes logic possible, and it was sitting unused in telegraph offices for eighty years.

2

Wire AND, OR and NOT from contacts

Three circuits, and every digital device ever built is made of them.

  1. AND: wire two relay contacts in SERIES between supply and lamp. The lamp lights only if both coils are energised.
  2. OR: wire two contacts in PARALLEL. The lamp lights if either coil is energised.
  3. NOT: use a normally-CLOSED contact. The lamp lights when the coil is NOT energised.
  4. Build all three and fill in a truth table for each by trying every input combination.

Series is AND, parallel is OR, normally-closed is NOT. That is Shannon's entire correspondence, and it is worth pausing on how unreasonable it is that a topology of wires should match an algebra of propositions.

Now the payoff: because it IS algebra, you can simplify circuits symbolically. De Morgan's laws say NOT(A AND B) equals NOT A OR NOT B — which means the same function can be built two different ways, and you pick whichever needs fewer contacts. Engineers had been optimising relay circuits by intuition; after 1937 they optimised them by algebra.

Mga materyales para sa hakbang na ito:

ReleRele4 piraso
Set ng LED na pahiwatigSet ng LED na pahiwatig1 set
Kit ng RisistorKit ng Risistor1 kit
Perfboard / ProtoboardPerfboard / Protoboard1 piraso
Kawad na pandugtongKawad na pandugtong1 reel

Mga kailangang kasangkapan:

Estasyon ng PanghinangEstasyon ng Panghinang
Digital na Multimetro na Pang-laboratoryoDigital na Multimetro na Pang-laboratoryo
Mababagong Pinagkukunan ng Kuryente sa MesaMababagong Pinagkukunan ng Kuryente sa Mesa
Malinaw na Salaming PangkaligtasanMalinaw na Salaming Pangkaligtasan
3

Build the same logic as blocks

Blockly Workspace

Loading Blockly workspace...

Mga kailangang kasangkapan:

Desktop na KompyuterDesktop na Kompyuter
4

Simulate, simplify, and count the contacts

Naglo-load ng Jupyter notebook…

Mga kailangang kasangkapan:

Desktop na KompyuterDesktop na Kompyuter
5

Measure the switching time, and find the ceiling

Relay logic works perfectly and is slow. Measure exactly how slow, because that number ends the era.

  1. Drive a relay coil with a square wave and put the oscilloscope on both the drive and the contact output.
  2. Measure the delay from coil energising to contacts closing, and again for release.
  3. Watch the contact closure closely at high sweep speed.
  4. Chain three relays so each drives the next, and measure the total delay.

A relay takes something like 5 to 20 milliseconds to switch, and the contacts BOUNCE for a millisecond or two after closing. Chain them and the delays add, so a computation passing through fifty gates takes most of a second.

The bounce is its own problem: a single logical transition arrives as a burst of several, and any counter downstream counts them all. Debouncing exists because a mechanical contact does not close once.

So the ceiling is mechanical, not logical. The algebra is fine and the relay is honest; it simply has mass, and mass takes time to move. The escape is a switch with no moving parts at all — the vacuum tube, which switches in microseconds, and then the transistor, which does it without the heat. That is the rest of this batch.

Mga materyales para sa hakbang na ito:

ReleRele3 piraso
Kit ng RisistorKit ng Risistor1 kit

Mga kailangang kasangkapan:

OsiloskopyoOsiloskopyo
Bumubuo ng AlonBumubuo ng Alon
Digital na Multimetro na Pang-laboratoryoDigital na Multimetro na Pang-laboratoryo
Mababagong Pinagkukunan ng Kuryente sa MesaMababagong Pinagkukunan ng Kuryente sa Mesa

Mga Materyales

5

Mga Kinakailangang Kasangkapan

7

Kaugnay na Blueprint

Ang mga blueprint na ito ay nagbabahagi ng kaalaman — mga teknik, materyales, o prinsipyo

The Telegraph Relay

The Telegraph Relay

ni EdFI

Electronics

40
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2
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Boolean Algebra

Boolean Algebra

ni EdFI

TEKNOLOHIYA

31
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0
0
0
0
Binary Arithmetic

Binary Arithmetic

ni EdFI

TEKNOLOHIYA

31
0
0
0
0
0

CC0 Pampublikong Domain

Ang blueprint na ito ay inilabas sa ilalim ng CC0. Malaya kang kumopya, magbago, mamahagi, at gumamit nang walang pahintulot.

Suportahan ang Maker sa pamamagitan ng pagbili ng mga produkto sa kanilang Blueprint Komisyon ng Maker itinakda ng mga Vendor, o lumikha ng bagong bersyon ng Blueprint na ito at isama bilang koneksyon sa iyong Blueprint upang ibahagi ang kita.

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