Bob Alexander's "Decimal Computer" Adds 1970s Technology to a 1960s Design

Maker Bob Alexander has dug deep for his latest computer build, and in doing so eschewed the modern fad for binary devices incapable of counting beyond two without duplication — by building a "decimal computer" first shown on the cover of Electronics Illustrated in 1966.

"Despite the magazine's hype of it being a computer," Alexander admits, "it's really just an adding machine. But I liked its retro look and the way numbers are entered with a telephone dial. The 'computer' used ring buffers for counting. Neon lamps turn on at higher voltages than they turn off at. This allows the lamps to act as switches and memory locations. It’s possible to build a circuit, without any transistors, where a neon lamp is lit, and a pulse from the telephone dial causes it to turn off, and the next lamp to turn on."

The only problem: the instructions, published as the cover feature in Electronics Illustrated's November 1966 issue, require components that are a little hard to find these days — like NE-2 neon lamps, long out of production, to be installed in a Bud Industries C-1588 enclosure. Worse still, even new-old stock may not work: "not only are the lamps all different from each other," Alexander explains, "but their behavior changes as they age."

Alexander's solution was to jump into a time machine to the 1970s, picking up some technology that would make the design more reliable and robust while still keeping the aesthetic of the original — adding solid-state 74HC192 decade counters and 74141 binary-coded decimal (BCD) decoder chips to drive the neon lamps, rather than relying on the lamps' unpredictable behavior directly.

"How does the article justify its claim that you can subtract, multiply, and divide with this adding machine," Alexander asks. "Well, subtraction is done using 9's complement. For the others, you still need to remember your multiplication tables, and the adding machine just assists you with the adding and subtracting."

The build is documented in the video above, and on Alexander's website; a PDF scan of the magazine from which the design was taken is available from the World Radio History site.