The November 1966 issue of Electronics Illustrated has a feature article, “Build This Space-Age Decimal Computer. I was inspired to make one of my own.
Despite the magazine’s hype of it being a computer, it’s really just an adding machine. But I liked its retro look and the way numbers are entered with a telephone dial.
My top priority was to make it look like the project in the magazine article. I wanted to use the same neon lamps (NE-2) and enclosure (which was not identified in the article, but turned out to be the Bud Industries C-1588).
As I researched the design described in the article, I realized it was not going to work. I remember, from my youth, hearing complaints that projects in electronics magazines did not work. I assumed it was due to errors in the article, or the fault of the person building them. But in this case, it’s simply an unreliable design.
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 problem is that NE-2 lamps are not manufactured for uniform on and off voltages. Not only are the lamps all different from each other, but their behavior changes as they age.
That means that to get a neon ring buffer to work with NE-2 lamps, you need to characterize each lamp’s behavior, group similar lamps together, and compensate for differences with resistors. After all that effort, if you get your adding machine to work, it’s still going to break after being used for a while.
Back in the ’60s, there were some neon lamps manufactured to more exacting specifications for calculating machines. But they were more expensive than NE-2s. There are even some NOS lamps available today that would work – but they’re twice the size of NE-2s. I had to choose between making my decimal computer match the original on the outside, or on the inside. I chose the outside.
(BTW, Pieter-Tjerk de Boer built an excellent Nixie clock using neon ring counters. His first attempt used NE-2 lamps. After a while, it stopped working, because the NE-2s changed as they aged. He built a new version using the more reliable MTX-90 – aka MTH-90 – lamps.)
So instead of using 1960s technology, I used 1970s technology. I used 74HC192 decade counter ICs and 74141 BCD decoders capable of driving the high voltages of neon lamps (and Nixie tubes!).
How does the article justify it’s claim that you can subtract, multiply, and divide with this adding machine? 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.
Check out the demo:
