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Building an 8-Line Automatic Telephone System

dial phone attached to 8 line DIY telephone system

At age 16, I was fascinated by telephone systems. I wondered, "How could my home phone dial any other telephone in the world?" This big question started me on a quest to find the answers. 

 

Also contributing to my interest was my local Panel Telephone Exchange in San Francisco (KLondike 2 phone prefix). This 1937 art deco building had a beautiful brick façade with huge windows allowing outsiders to view the exchange workings (imagine that being allowed today).

Here are two closeups of the sorts of things I was seeing through the windows. Image from E.B. Craft, Journal of AIEE, April, 1923

 closeups of Panel telephone system banks

         Vertical selecting rods, sequence switches, clutches, brushes, terminal bank

Some of the vertical rods were randomly ascending and descending, motors were turning, other parts were intermittently rotating. I had no idea what I was seeing. It made a lasting impression on me. I discovered years later that the moving vertical rods had electrical wipers (brushes) that made contact with a panel of stationary contacts. The wipers were directed to stop on a particular set of terminals and this helped establish the "talking path" between 2 phones. 

At times I would tap on a building window and gesture, “Please let me in”. I was always waved off. Yet somehow this fantastic collection of electromechanical gear held the answers to my big question. It made the quest even more exciting.


Another clue to the puzzle was a 4-digit Step-by-Step private telephone system (sometimes called a PBX, Private Branch eXchange) located at UCSF hospital and medical school, walking distance from my childhood home. Step-by-Step was a widely used system for making phone calls.

 

After worming my way in, a friendly repair technician helped me appreciate that behind every phone number there were two dedicated relays; one called Line (L) and the other called Cutoff (CO). Grasping the purpose of these two relays was a profound revelation and an essential part of the answer to my big question. 

step by step telephone system switches strowger

Step-by-Step (Strowger) switching systems

I also learned by watching the UCSF PBX in action.  I enjoyed tracking a caller's dialing progress through switches, namely Line Finders, Selectors, and Connectors (as shown in the pictures above).  This was especially true when, occasionally, only one person was making a call. In this case, it was easier to see and hear the call's progress through the switch train. This knowledge started me thinking that maybe, just maybe, this kid could construct a basic relay-based small PBX in his workshop. Of course, I did not have access to individual Step-by-Step switches so I needed to find an alternate switching method.


Over time, through trial-and-error experiments, I gained an understanding of how to build a very simple rotary stepping system, distinct from the much larger SxS and Panel systems.  A plan was drafted to build a small single digit, 8-line, automatic dial system. No better way to learn than to build a DIY phone system. See below for more on the rotary system design details. 

While this small system won't connect millions, it could be scaled for thousands and then replicated geographically. By interconnecting many such systems a universal network would be possible.


At age 17, I was fortunate to be a student of Carl Koenig, physics teacher at Lowell High School (San  Francisco).  He was an enthusiastic champion of DIY projects and he helped procure the control relays and stepper switches for my planned automatic telephone system. Also supportive was my grandfather, a home builder and excellent craftsman. He constructed the wooden frame (and a table) to mount all the components. So, all the parts were donated and it mainly cost sweat equity to build it. 


From start to finish it took about 18 months to complete. The parts were assembled, wired, and tested in the same order as a call’s progress. The construction order was:

•    First give the caller dial tone (assisted by a rotary Line Finder switch).  The rotary stepping switch is described later in this article 
•    Then respond to the dialed single digit and advance the rotary Selector switch accordingly 
•    Then ring the called phone if not busy 
•    Then provide a talk path between the caller and called phones.
•    Then on hang-up, release all activated relays and reset the Line Finder and Selector rotary switches to home position 

 

Looking back, it was exciting to see the system “come alive” literally step by step. I will never forget the first time the system provided dial tone to the caller. What a happy day for this kid. 


I spent many long weekends working on the design and construction. Once around 2 AM in the morning, while working on the system in my workshop, my dad hollered out to me, “Come to bed right now!” Ah, the love of making things! 

At age 18, I entered the system into the annual San Francisco Bay Area Science Fair held at the Science Museum in Golden Gate Park. The system was what today we call a “STEM maker project”.

The figure below shows the completed system front and rear views (3’ 6” tall, without table) and was the version submitted to the Science Fair. 

front and rear sides of DIY 8 line automatic telephone system

                      8-line, single digit, automatic dial phone system --2005 image

                                (Soundproof relay box removed for this photo)

The system was stored in my childhood home workshop for ~50 years. It was operational for the first ~30 years then a big water leak damaged it. Currently it is non-operable and under repair. 
 

For more detail on this DIY rotary stepping switch design, with pictures and videos, go to under the hood. 

Epilog
 

A few executives from the Pacific Telephone and Telegraph Company (PT&T) toured the Science Fair that year and witnessed the phone system in operation.  Soon thereafter, I was offered a position working in the Chief Engineer’s Laboratory -- under the watchful eye of Don McLeod, an extraordinary telephone engineer.  The work experience changed my life. 

Pursuing the answers to my big question had some wonderful and unexpected payoffs.

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