Here Come the Relays – Clickety Clickety Click…

1-Bit Relay Adder

1-Bit Relay Adder

Now that we have a Twitter Arithmetic Logic Unit, we can begin the fun stuff! Although 7400 logic is a classic platform for logic circuits, there is still an air of mystery about what goes on inside those little black chips. The advantage of 7400 logic is that it’s incredibly easy to use and the variant that we’ve chosen (74HCT) is super speedy.

But don’t worry, the Twittithmetic Logic Unit (TLU) isn’t just a PCB with a few black boxes. The core aim of the TwitALU project is to make computing rhythmically audible and visually entertaining. This is where the adder module comes into its own.

As you may remember, Jeremy and I came up with the idea of a core “kernel” and modular “modules” that can be swapped in and out at will. The TLU is effectively the kernel, but we can now explode some of those 7400 logic chips into slow, clunky, loud, inefficient relay logic boards – going as deep down into the computation as we dare.

Relay Adder Schematic

Relay Adder Schematic

I’ve spent the past couple of days rebuilding the 8-bit full adder using relays. I used our favourite circuit design software tool (Proteus ISIS) to draw up the schematic, simulate the circuit, and then layout the PCB using Proteus ARES. My initial estimates showed that we would need 80 mechanical, SPDT relays to make an 8 bit full adder. However, through many optimisation (and a bit of trial and error) I have managed to reduce this to 24 – just 3 relays per bit!

8-Bit Relay Full Adder

8-Bit Relay Full Adder

8-Bit Relay Adder Rear

8-Bit Relay Adder Rear

So, the idea is that we can simply unplug the 7400 logic full adder IC and plug in our relay based version. This is great as it shows that our relay implementation performs exactly the same as the chip. To make it even more exciting, we will be using clear-cased SPDT relays and outfitting them with 3mm super-bright LEDs.

Omron G2R-1 5DC Relay

Omron G2R-1 5DC Relay

My next task is to order a few of the relays and LEDs to make up some prototypes, thus ensuring that our modifications are possible. We may also run into problems with switching the relays as they’re particularly power hungry (47Ω, 5V coil). Some buffer chips may be needed to drive the coil of the relays.

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One thought on “Here Come the Relays – Clickety Clickety Click…

  1. A/P Daniel F. Larrosa (Montevideo - Uruguay)

    Nice job and interesting idea, very useful to teach the basics of digital logic by showing the gates “moving”.
    Question: I think the relay adder can be simplified by substituting a double selector relay for RL1-RL2 (and RL4-RL5, etc), as RL1 and RL2 operate simultaneously.
    Did you did it that way on purpose ? (I think, may be it was a design choice, to make the circuit more modular, by using always the same type of relay or to make it more “logic-gate-like”). Thanks for sharing.

    Reply

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