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Recently I was dismantling an old (built in 1967) Hewlett-Packard Model 312A Wave Analyzer. Despite the fact the machine was built from discrete transistors it had a digital frequency counter inside. The counter features Nixie tubes: a neon tube where the numbers to be displayed are actually metal electrodes shaped like the number to be displayed. Making the electrode several hundred volts negative relative to a backing electrode causes the numeral to appear as a glowing digit.
For a complete description of how Nixie's work, see the description on the Nixie Tube Page by Tom Jennings
I was pondering just HOW they got the actual counter to work. Examining the counter boards there are no IC's, no BCD counter chip, no decoder chip (like the familiar 7441), just eight lowly transistors. The counter itself appears to be a simple binary counter built from eight discrete transistors wired as four flip-flops. Simple, but effective. So then, how did they decode the count to display it on the Nixie? That is by far the most intriguing piece of circuitry in the unit!
A single digit counter board is pictured here. The counter itself, with the eight discrete transistors (The small metal cans) wired as flip-flops is seen on the right side of the board. The Nixie display tube is on the far left in a socket (It is built by Burroughs). The 'black box' behind the Nixie is the 'magic' decoder.
Taking that black-box apart reveals an ingenious opto-isolator consisting of multiple photoresistors, produced in thick-film, on a ceramic substrate. The photoresistors are illuminated from below by eight neon lamps which form the output of the counter, two neons per stage (one Q and one Q-bar output for each counter). Decoding is accomplished by lighting neon lamps from the counter. This illuminates several photoresistors on the substrate. Depending on which resistors are illuminated (They form series chains) one of the Nixie's shaped electrodes is made negative 170 volts while all others float positive so that a single numeral is displayed.
In this close-up you can see the actual photoresistors. I have drawn an outline
on top of the substrate to show where light from the neon lamp illuminates. In the
case of the first lamp, it illuminates both R1 and R2. The second neon lamp
illuminates R3 and R4. Each neon lamp illuminates either two or three individual
photoresistors.
I was recently sent a schematic diagram for the counter circuitry from a reader in the Netherlands and have included it below as a PDF. The diagram below shows how photoresistors are wired in series to select a single digit given any pattern for the neon lamps. Click on the diagram of the photoresistors to show the schematic for the entire counter board.
An absolutely INGENIOUS way of using common 60's technologies (neon lamps and photoresistors) to accomplish a reasonably complex decoding scheme! A decoder based on transistors would, at the time, have been much more complex, and expensive, to build.
Many thanks to the reader who sent me the original schematics for the counter as well as another who sent me a spare counter ... these will come in useful when I (finally) get around to building a clock using these (I will post the design here when complete but don't hold your breath ... its on the "low priority" list :). .
If you're wondering WHY a person accustomed to working with the "latest and greatest" technology wrote this page ... I have always liked the look of a soft, glowing Nixie display. My (cherished) workshop frequency counter is an old General Radio model 1192, bought in 1991 at a University surplus sale for $20, which uses Nixie's and still works great.
Judging from an article in the IEEE Spectrum (June 2002), as well as the number of people who have built nixie-tube clocks, these glowing tubes have made quite a comeback in nostalgia circles!