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Telephony Projects at home:
555 Switchboard (Cordboard/Plugboard)
Step by Step PBX Exchange

Since the late 19th Century, Central Office telephone exchanges have provided telephone service for local subscribers. Rotary Dial “Strowger” switches (automatic exchanges) became widely employed in the 1930s and 1940s. In my hometown, we finally got Rotary Dial in 1964, having previously had (human) operator-staffed “Number Please” (manual service) switchboard offices.

Touchtone (DTMF) started replacing rotary dial in the Bell System in the 1960s.  The manual plugboard switchboard was made famous by comedienne Lily Tomlin and her antics as an operator.

In our PBX, we use Western Electric step-by-step switches invented by  Almon Strowger, whose funeral home undertaker business was losing clients to a competitor whose telephone-operator wife was redirecting everyone who called for Strowger to her husband’s business. Motivated to remove the intermediary human operator, he invented the first automatic telephone exchange in 1889 and received its patent in 1891.

Rotary Dial PBX (SXS, step-by-step Strowger stepper switches)
Linefinder shelf in our PBX

Why this project? everyone asks

Growing up, I was a telephone nerd, and at age 14 bought my first stepper switch. Without any documentation or instruction, I was able to get it to be somewhat operational, inelegantly mounted hanging at the edge of a stepstool. During high school, in parallel with rebuilding a pipe organ, I bought several rotary stepper switches and added some relays to make a rudimentary 2-phone dial system.  Now, upon nearing retirement, the telephone bug resurfaced, and I started collecting SXS (Strowger step-by-step) switches, and related components to construct a PBX exchange.  Ebay was my friend!

SXS Strowger stepper switches
Initial collection of steppers, guarded by Midnight the cat

Now 90% complete and mostly operational, it currently serves as a conversation piece for guests, and eventually will be donated to a museum to demonstrate this technology.

Features of the PBX

  • 4 digit extensions (subscribers)  (7 digit with digit-absorbing: first 3 digits are ignored)
  • 8 links (simultaneous conversations)
  • 200 point line finders
  • Additional demonstration technology: AE-style rotary stepper linefinder, and per-station rotary stepper hunt for free selector
  • Dial 8(Panasonic 612 PBX), 9(outside trunk line(PTSN)), 0(Operator: 555 Switchboard)
  • Two ringing cadences (not synchronized)
  • Two intercept recordings (selector, connector)
  • Dialtone (Imprecise tone: free-running 350 and 440Hz oscillators), busy signal
  • Rotary hunt, level hunt for busy stations
  • Tone to pulse converter (prior to first selectors)
  • Dashboard status lights
  • Power (48, 12, 5, 100VDC(for coin pay station), and 30 Hz ringing current from frequency generator)
  • Aisle supervisory lights
  • 1A2 multi-line “business” phones
  • 3-slot pay phone trunk
  • Python script (running on a Raspberry Pi) to simulated eight stations randomly dialing (over and over).  This serves as a great demonstration for the drama of the relays chattering, as well as a debugging tool (the script keeps history for replaying if needed to recreate a problematic condition)
  • Future integration with C*net (internet phone network)

Timeline of Current PBX Project



Unusual Techniques Utilized