Electronics Archaeology: Building a Relay-Logic Line Cord Tester in 1978

By Cliff Potts, CSO, and Editor-in-Chief of WPS News

Baybay City, Leyte, Philippines — July 17, 2026

A few years ago, I realized that I never really finished my electronics education.

I attended Prosser Vocational High School in Chicago and studied electronics, but life took me in other directions. That realization is one of the reasons I started the Electronics Archaeology series. It gives me a chance to revisit the things I learned, the things I forgot, and the things I experienced while working with electronics nearly fifty years ago.

After discussing magnetism and relays in the previous installment, this seems like the right time to tell a story from 1978.

Back then, I was nineteen years old. I had a GED and an unfinished electronics education. Looking for work, I rode a bus out toward Chicago’s Northwest Side and started walking from shop to shop along Northwest Highway.

My job search consisted of a single question.

“Are you hiring?”

One of those shops was J. Phillips Industries, a small manufacturing company located near Northwest Highway and Nagel Avenue. The owner hired me to drive a company van and pick up parts around the Chicago area.

That was the job I was hired to do.

It was not the job I ended up doing.

A Production Problem

At some point, the company needed a better way to test electrical line cords before they left the factory floor. The existing process was too manual and too slow for the production environment.

The cords contained multiple conductors that had to be connected correctly. The workers needed a simple system that could guide them through the testing process and immediately identify wiring mistakes.

Today, the entire project would probably be handled by a small microcontroller costing only a few dollars.

In 1978, I built the solution with relays.

Magnetism at Work

A relay is one of the most practical applications of magnetism in electronics.

When electricity flows through a coil, it creates a magnetic field. That magnetic field pulls a mechanical armature, which opens or closes electrical contacts (Horowitz & Hill, 2015).

The line cord tester relied on dozens of those magnetic switching actions every time it operated.

The system used a transformer to reduce line voltage to approximately 48 volts AC. A bridge rectifier converted the AC into DC, and filter capacitors smoothed the output enough to operate a bank of 48-volt DC relays reliably (Kuphaldt, 2020).

The tester guided operators through a sequence of checks. Indicator lamps told them which conductor to test. If the correct wire was presented to the sensing plate, the machine confirmed the result and advanced to the next step. Time-delay relays held the indication long enough for the operator to see it before moving forward.

Everything was built from scratch.

The relay logic.

The power supply.

The circuit boards.

The wiring.

Every piece of it.

When Reality Votes

The first version was not perfect.

At one point, I created a relay-logic condition that established a direct short through part of the system. When that condition occurred, the resulting current flow damaged a significant portion of the relay chain.

The machine had found a flaw in my design.

That is what prototypes are supposed to do.

During the rebuild, I improved the power supply and continued refining the design.

I also made another mistake that many electronics technicians have made at least once in their lives.

I installed an electrolytic capacitor backwards.

The capacitor immediately demonstrated why polarity markings exist.

Fortunately, the lesson was far more dramatic than it was expensive.

What Engineering Really Looks Like

One thing that has always amused me about discussions of engineering failures is how surprised people seem when prototypes fail.

Whether the project is a relay-based tester in a small Chicago manufacturing shop or a modern aerospace vehicle, prototypes exist to reveal problems.

A design that has never failed may simply be a design that has never been tested.

The goal is not perfection.

The goal is learning.

Every failure answers a question.

Every redesign improves the system.

Every mistake teaches something that can be applied to the next version.

That was true in 1978, and it remains true today.

Looking Back

Eventually I left J. Phillips Industries and went to work for the Illinois Toll Highway Authority as a repair technician working on relay-based automatic toll equipment.

What happened to the line cord tester after I left, I honestly do not know.

I handed over the schematics, notes, and hardware and moved on with my life.

Once I left the company, I never looked back.

Nearly fifty years later, however, I still remember the machine.

I remember the transformer.

I remember the bridge rectifier.

I remember the capacitors.

I remember the relays.

Most of all, I remember being nineteen years old, hired to drive a van, and somehow finding myself designing and building a custom industrial test system.

The company is gone.

The machine is almost certainly gone.

The lessons remain.

That, in the end, is what Electronics Archaeology is all about.

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References

Horowitz, P., & Hill, W. (2015). The art of electronics (3rd ed.). Cambridge University Press.

Kuphaldt, T. R. (2020). Lessons in electric circuits (Vols. I–VI). Open Book Project. https://www.ibiblio.org/kuphaldt/electricCircuits/