Logic Gates from Relays

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.

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.