Relay Resurrection

The restored relay clock, in its original glass case

I have renovated the components of my nearly 50-year-old digital clock. The next step was to assemble it all back together. Would it actually work?

The old broken and abused internal plexiglass chassis was replaced by new plexiglass, providing an opportunity for me to learn the technique of plastic welding, where a syringe injects solvent into the edge of a surface-to-surface joint and spreads by capillary action to the full contact area, partially dissolving the plexiglass, which then forms new polymer bonds between the pieces. It takes a few minutes for it to start hardening, which gives some time to prop the parts in the desired position (use a square to get the angles right). It is completely cured in 24 hours and is truly “welded”. Like a good metal weld, a good plastic weld will break elsewhere if enough force is applied.

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The Relay Clock Display

Digital readouts, probably from an aviation display

I recall seeing displays similar to this in elevators when I was very young, but it appears that these digital readouts came from a cockpit display or some other instrument. It seems rather impractical to me today, but digital displays were difficult to make back then, especially for the rugged environments found in aviation. I found a display similar to this being offered at a surplus site.

A front and rear view of a surplus aircraft readout. There was only one available, at a price of $150!

The basic idea is that there are ten light bulbs for each display digit. One of them is energized and lights up. It projects a numeric image onto a screen.

In this clock, the relay contacts direct a voltage to select a display digit. The relay coils operate at voltages of 12V, 24V, and 110V, but the display uses light bulbs that run at 6.3V, a common voltage used for vacuum tube filaments and pinball machine lights. You can see why 6.3 was a popular voltage, right?

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The back end of two three-digit display modules. On the right, the bundled cable wiring has been replaced by flat ribbon cable with insulation displacement connectors. The remnants of the old ribbon cable from the relays are seen on the left, individually soldered to the display lamp wires.

I had planned to replace the inadequately designed power supply for the clock, and I had figured out how to update the signals to the relay coils, but I had really hoped that I could avoid re-wiring all of the individual connections between the relay contacts and the display bulbs (10 + 6 + 10 + 6 + 10 + 2 of them).  I had figured out the connections and how they could be used with the new power supply without having to completely rewire them.

In 1973 I was using some of the latest technology, including “ribbon cable”, an evolutionary step from a tied cable bundle.  Individual wires were laid side-by-side and cast in place with an insulating plastic bond.  They were also called flat cables.  Once again, my source of this unusual wiring system was from my dad’s ham radio shack. 

I found them particularly appealing because they were color-coded with the series used to identify resistor values-  black, brown, red, orange, yellow, green, blue, violet, gray, white to represent digits 0, 1, 2, … 9.  They include the colors of the rainbow, and I recall thinking how nice they will look in the finished clock, which motivated me as I connected them to the stepper relays. 

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Early Integrated Circuits

Two 7490 integrated circuits. The date code tells us that they were manufactured in September of 1972.

Although I was unaware of the new integrated voltage regulator circuits when I built this clock, I was familiar with an early series of integrated logic circuits, the 74xx TTL chips (TTL stands for “transistor-transistor-logic”). Again, the ability to replace multiple circuit boards of discrete components that implemented a specific logic function with a single integrated circuit chip, was revolutionizing electronics.

An example is the 7490, a small 14-pin device that implemented logic that could count to ten. I used two of them, configured to count to 60 and then start over. I fed it a clock input that was derived from the household AC line, 60 cycles per second, and it delivered a logic pulse once a second to the first relay in the clock.

I wanted to keep this relic of a circuit in the renovated clock, and so I adapted its input to the AC signal from the new power supply. But I had forgotten the rules for using the ancient TTL logic, which required much higher current than is used today. Modern CMOS logic uses almost zero electrons to do their magic, which is why your phone doesn’t discharge within a few minutes, blistering your hand with the heat.

My first attempt to trigger the old timekeeping logic resulted in paralysis. No ticks.

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The Relay Clock

It may not look like it, but this is a clock, one of the world’s first digital clocks, circa 1973.

When I was twenty or so, I built a clock. It was a clock that used electromechanical relays to count the seconds, minutes and hours. I was early in my electrical engineering studies and barely understood Ohm’s law, but I was aware of electromagnets, and how they could be used to make mechanical movements. Some of us call them solenoids, coils of wire that magnetically push or pull an iron piston to make and break switch contacts.

My dad, in his ham radio hobby quest for cheap electronic components for whatever future project he might undertake, had acquired various relay mechanisms. He showed them to me and explained how they used electrical pulses to move a metallic wiper to a series of successive contacts. These were telephone relays, used when you “dialed” the phone. One relay, responding to a single rotary dial action could connect you to ten possible neighbors. Cascading it with another set of relays would allow calling up 100 neighbors. More phone digits would extend it even further, you get the idea. I barely understood how to deliver current to energize a relay coil, but I could figure out how to connect the contacts to make a sequence that counted.

Another item from my dad’s ham shack inventory of salvaged components was a numeric display, something you might see in an elevator (60 years ago) indicating the current floor. It used incandescent light bulbs to project a numeric figure on the screen. Ten separate bulbs, each behind a number glyph mask; one of which was selected for the current digit value. I figured out how to connect the telephone relays to the elevator displays to make a clock.

I was quite pleased with the result and when I got it working, I showed it off to my friends. But it had several drawbacks. First, it was noisy. Relays are mechanical devices that magnetically pull metal contacts against each other, resulting in a click-clack noise as they activate and release. The relays counted seconds, so there was a click-clack noise every second. And at ten seconds there was an additional click-clack, as the next relay responded to the carry pulse. And at 59 seconds, there was a carry to the minute-counting relay. As the carries propagated to the hours, a peak acoustic disruption occurred as the clock transitioned from 12:59:59 to 01:00:00.

The other drawback was that the clock got hot. As I said, I did not yet understand the relationships of voltage, current, power and heat. I just hooked up the components to make the displayed result I wanted. The numeric display, comprising light bulbs, used quite a bit of power. And the relays required power too, and my power supply was terribly inefficient. As a result the clock, in the glass case I made for it, built up an enormous amount of heat. I tried to ameliorate it by installing an internal fan, but this seemed only to make things worse (the fan used power too).

I kept the clock as a curiosity piece, displayed on a fireplace mantel in our home. It was intolerable to run continuously, so I would switch it on to show visitors that it actually worked, and then shut it down to stop the noise. Eventually, the clock got packed up during one of our moves and stayed so.

The boxed-up relay clock was in storage for at least 30 years, but I would occasionally encounter it hiding among my workshop parts and supplies. I recently ran across it. It had endured the desecration by (and excrement of) rodents, the decay of electrical components, and the binding of mechanical joints. I wondered why I had saved it all these years.

Well, the idea of resurrecting a contraption that I built half a century ago carried a certain appeal to me. Having since learned the principles of electricity, maybe I could bring it back to life and its former geek glory!

In the next series of posts, I will describe the process of restoring this pioneering clock.


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Modern Power Supplies

State-of-the-art power supply in 1973 next to a modern (for over 40 years now) voltage regulator.

As I mentioned, I was not skilled in power supply design in 1974. I am still no expert, but I have acquired some familiarity with them over my career. Back then, one needed to understand transformers and bridge rectifiers and capacitors. They were simple, but limited.

Think of your basic plug-in wall charger. A few years ago, they were made of a dense and bulky transformer, diodes, and a capacitor, the smallest components that the designer could find to meet the power requirement. They were not terribly efficient and could only provide a few watts.

Today your cell phone and computer chargers can deliver hundreds of watts but are smaller and lighter weight than those old “wall warts”. They are the beneficiaries of new power supply technology that uses high frequency internal circuits to replace the old iron cores of the 50-60-cycle transformers.

I could now replace my old 24V center-tapped transformer-based power supply with a modern “AC to DC converter” that could provide more power at higher efficiency than was possible back then.

I needed some more voltages: 12V and 5V. The old supply took the “center tap” of the (24V) transformer to provide 12 volts for those relays that needed it. The 5V for the logic circuit that generated the one-second pulse was obtained by a crude arrangement of diodes and resistors powered from the 12V line. I’m amazed it worked.

But that was what was available back then: diodes and resistors. Today it is trivial to generate stable power supplies by using the ubiquitous 78xx series of voltage regulators, a component that has three pins: voltage-in, ground, and voltage-out. These breakthrough parts were first manufactured in the early 1970s, a time when “integrated circuits” had recently been invented and were being applied to an ever-increasing number of applications. In this case, elaborate voltage regulation circuitry that had previously required dozens of discrete components were now implemented by microscopic semiconductor junctions contained on a single “chip”. At the time I built this clock, regulator chips were becoming available, but I did not yet know about them.

Today (and for the last 40+ years), I use the 7805 to provide a +5 volt supply, and a 7812 to generate +12 volts. This will be part of my power supply renovation.

AC to DC converters (black modules) for the renovated clock. One provides 24V, the other supplies the 6.3V for the display lamps. The object plugged on top of the power cord is an isolation transformer that provides the primary timing signal.

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A closeup of the ten-step relay. It moved sluggishly and got held up on the tenth step when the carry contact (upper right) could not close, stopped by the bump on the circular cam.

I re-wired the relay coils for the improved control circuit and hooked up the new power supplies. A set of micro-switches were used for setting the time—they momentarily applied power to the relays with each button push. I could now see if the relays still worked. They did!

Well, they mostly worked. The contacts had tarnished and needed cleaning, and some of the mechanisms got stuck in one or more positions. I applied the usual treatment for things that stick—WD40, but it was not enough. The lubricant that finally allowed the decade stepper relays to move freely was something called “Nano-Oil”, a substance using “Magnetically induced Molecules of 0.09 microns”, that my son had gifted me a few years back. At the time I wondered what I would use it for, but he evidently saw my future need for it.

“Nano-oil” helped to re-lubricate the moving parts on the relays.
A short video showing the (restored) relay activation and contact movement.

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Holiday Card History

My grandparents’ Christmas card, circa 1950

I grew up watching my father, following his father’s example, coming up each year with handmade Christmas cards that nearly always included a family photograph.  They were both avid amateur photographers and would corral and cajole my siblings and me into a studio-like set in the living room with carefully positioned lights and a camera mounted on a tripod that would be aimed at a scene of dressed-up children surrounding their proud parents.   This often occurred at our family Thanksgiving gathering, allowing just enough time for my mother to get prints made, mounted into cards (often with her hand-stenciled or stamped cover designs), personal greetings inscribed, and envelopes addressed and stamped, all before the week of Christmas.

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Indirect Endowment

[I write this not to gain credit or accolades, but as an attempt to inspire others who may have been blessed by similar good fortune or have been more successful than expected in saving for their futures to consider what to do with their “excess”.] 

My dad once told me that he was planning to “spend his children’s inheritance”.  It was his lighthearted way of saying that he was not going to restrict his spending during retirement.  He intended to pursue his passions for inventive projects and for philanthropic activity, especially for educational causes.  And that his children should continue saving for their own financial security.  None of us expected any different.

Well, he failed.  Despite his efforts to create the ultimate ham radio station, and to support his grandchildren through college, he left a surplus.  Not a Warren Buffet or Bill Gates level of wealth, but certainly more than we expected from a man who worked for a salary and who, while we were growing up, paid the mortgage by keeping our daily expenses to a minimum.

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Dad’s Dressing

With apologies to Newman’s Own

My mother managed a large household in a small house on a tight budget.  She had five children within a decade during the 1950s.  Armed with her Better Homes and Gardens cookbook, she prepared dinner every evening for a table of seven and had it ready by the time Dad came home from work.  Diet and meal recommendations in the US at that time were meat, starch, vegetable.  The food pyramid was yet to be invented, but family traditions provided the same guidance.

Our family dinners were always accompanied by a green salad: iceberg lettuce, carrots, celery and tomatoes, tossed into a large bowl and passed around the table for us to fill our individual plates or salad bowls.  My mother’s care in stretching her self-imposed grocery budget resulted in a few interesting dinner rules, one of them being that no one was permitted to take more than two slices of tomato in their salad portion.  (Another consequence of her frugality was that frozen orange juice was diluted with an additional measure of water, permitting us all to have a full juice glass at breakfast).

There was only one option for dressing the salad.  My mother would never consider buying those expensive bottled dressings because my dad could make a French-like salad dressing at home.  We would often watch him do this just before dinner.  A bottle would be fitted with a funnel, into which he would deliver various amounts of spices from the small spice jars kept on a lazy susan in the cupboard.  It was quantitatively uncalibrated; he’d just shake some into the funnel, give the lazy susan a turn, and see what else was available to add to the mix.  He would then add ketchup, followed by vinegar, oil, and water.  On occasion he would add drops of Worcestershire or lemon juice.  The bottle would then be capped and vigorously shaken to mix the ingredients into a tart and flavorful concoction that would eventually separate back out to something clearish floating over something reddish.  Shaking to remix the dressing before putting it on our salad was part of the dinner ritual as we passed the bottle around the table.

We all grew up with the understanding that dinner was always accompanied by a tossed salad, and there was only one dressing for it—Dad’s.  Over time of course, we grew up and left home and were forced to explore the commercial options for salad dressings.  They always seemed to come up short- too sweet or too thick or not enough tang. There was always some deficiency compared to Dad’s.  Later on, at extended family gatherings we would insist that he bring his dressing to provide an option at the salad station.  At some point, we pressured him to write down how to make it so that we could, in principle, reproduce it.

He yielded to that pressure.  To somehow quantify the arbitrary shakes into the funnel must have been an interesting exercise for him; it took a few iterations before he was satisfied.  He recorded his measures and supplemented them with an elaborate procedure to put them together.  This is not the technique I watched as a kid (which was quite simple– just shake the bottle), but the recipe at least gives a glimpse of how he thought it should be done.

I recently encountered that recipe and made a few batches of Tod’s Homemade Salad Dressing.  It was close, but did not match exactly my long-ago fond memories of it.  On the other hand, there may not be anything that would match those memories.

Realizing that perfect reproduction was impossible, I made an adaptation that scales the recipe to fit a standard size bottle and utilizes only two distinct measuring tools.  I invite you to try it.  Feel free to make any modifications you think might improve on it.  Dad would.

Tod’s Homemade Salad Dressing
Thor’s variation, using only two measures and fitting in a 16-oz bottle.

1 tsp    celery salt/ground celery seed
1 tsp    onion salt/onion powder
1 tsp    paprika
½ tsp   salt (fill the tsp measure halfway)
½ tsp    black pepper
½ tsp   lemon pepper
1 cup   vinegar, in two ½ cup parts
½ cup   ketchup
¼ cup   oil (fill the ½ cup measure halfway)
¼ cup   water

Add the spices to the bottle.
Add ½ cup of vinegar and shake to dissolve spices in the vinegar.
Add the ketchup, probably using a funnel.  Shake to mix.
Add the oil, rinsing the ketchup through the funnel.  Shake.
Add the remaining vinegar, rinsing the oil through the funnel. Shake.
Add the water. 

Lower calorie option:  omit the oil, increase the water.

Other optional ingredients to consider:  Worcestershire sauce, lemon juice, garlic…