The Simplest Electro-Mechanical Telephone Exchange That Actually Works

Several relays and switches mounted on a metal frame

While rarely seen by users, the technology behind telephone exchanges is actually quite interesting. In the first hundred or so years of their existence they evolved from manually-operated switchboards to computer-controlled systems, but in between those two stages was a time when dialling and switching was performed electromechanically. This was made possible by the invention of the stepping switch, a type of pulse-operated relay that can connect a single incoming wire to one of many outgoing wires.

Public telephone exchanges contained hundreds of these switches, but as [dearuserhron] shows, it’s possible to make a smaller system with way fewer components: the Cadr-o-station is built around one single stepper switch. Although it looks rather complicated, the only other components are a bunch of ordinary 24 V relays and a few power supplies. Together they make up a minimal telephone exchange that connects up to ten handsets.

It doesn’t have all the functionality of a larger system however, as only a single voice circuit is made to which all phones are automatically connected. Still, it does allow users to dial a number and let the other phone ring, which might be good enough for a home or indeed the hackerspace where it’s currently sitting. It’s also a fine demonstration of how relatively simple technology can be applied to make a surprisingly complex system.

[dearuserhron] wrote an in-depth article on the workings of electromechanical telephone exchanges, which might come in helpful to anyone who’d like to design such a system for their own home. For a more general introduction into analog phone technology, check out our analysis of a 1970s rotary telephone.

27 thoughts on “The Simplest Electro-Mechanical Telephone Exchange That Actually Works

  1. The coolest relays are the ones which stepped two ways. They’d step up for the first number, then over for the second. So they had like 100 contacts in one deck, and there were two or 3 decks. I forgot what they’re called, but a whole building full of them is pretty impressive. I worked in the main exchange in Detroit in 1966. One floor was full of 2 volt submarine batteries for backup.

    1. When I was a building contractor in the late 80’s I did some work for PacBell in one of their NorCal centers. I got the contract via a friend who took me on a tour of the facility. The one thing that still stands out for me was a 1+ acre building of wall-to-wall lead acid batteries to power the system. Interestingly, I don’t remember what I built…

      1. Was in a toll office back in the 60s and saw the ammeter on the main battery bank.
        The batteries were floating so the full current of 2000A was going to the office.
        So in a power outage those batteries had a 2000A load to feed for many hours

    1. Earlier than pinball computers I think, they’re a Victorian-era invention… the inventor, Stowger was an undertaker:

      Anecdotally, Strowger’s undertaking business was losing clients to a competitor whose telephone-operator wife was redirecting everyone who called for Strowger. Motivated to remove the intermediary operator, he invented the first automatic telephone exchange in 1889; he received its patent in 1891. It is reported that he initially constructed a model of his invention from a round collar box and some straight pins.

      https://en.wikipedia.org/wiki/Almon_Brown_Strowger#Rotary_dialing

    2. That one is called a uni-selector or a line selector or a hunter depending on what it did. It didn’t have any response to the hone dialling, it was in a different section of the exchange (schematic) lol, Well if you worked on this old technology then on some smaller exchanges the schematic charts had more surface area than the floor the exchange was built on.

  2. I’ve got a few of those stepper switches floating around somewhere. They’ve been in a few projects over the years. Lighting displays and high voltage thibgs mostly. It would be interesting to try and find a few real rotary phones. When they were more common, I would use them for parts. How things have changed. The carbon microphones were great for acoustic resonance experiments.

  3. There’s a museum in Seattle, near SeaTac airport that has 3 different telephone switches, all working, and they give tours…only on Sundays though. Well worth the visit if you’re in the area.

    https://www.telcomhistory.org/connections-museum-seattle/

    If you want your own PBX, you can get Panasonic KX-TDAxx pretty cheaply off eBay. I have a TAW824, but there are simpler ones. You’ll spend $100-$300 depending. Work with both tone and rotary phones, regular telephone lines as CO connections (or use an OOMA or other VOIP box)

    1. Just find a low voltage wiring company and ask if they can get you one. PBXs are being removed by the truckload and generally have zero value except for recycling. If you had asked me a year ago I could have given you several Nortel PBX systems.

  4. Early 90s, small rural midwestern town. They’re upgrading the local telephone exchange from Strowager Relays to digital systems. I set my little self down and watched the old equipment coming out and being thrown into a scrap truck, and two racks worth of computers get wheeled in. I wish I would have gone up and asked for some bits.

  5. We are still using Uniselectors at The National Museum of Computing at Bletchley Park, UK for the EDSAC rebuild. The ‘boot’ ROM is identical to the 1949 one with a set of uniselectors whose contacts are hard wired to 0’s and 1’s. As the uniselectors step round they put the boot code into the bottom 16 words of memory to load a paper tape with rest of the boot up code.

  6. What people today can never experience that the soundscape of being in an electromechanical exchange. I will give some pointers in re-creating this old technology simply further down.

    I used to service a variety of these technologies and when you understand how the whole system works you can be sitting in a room that is 15m x 15m and hear the chatter of all the relays and other electromechanical devices as the calls process through and understand what every separate click is and what connection it is making. You can even hear when a electromechanical device is preforming substandard and go find it and re-adjust it to specifications. It’s like seeing the Matrix digital rain and understanding it. It truly a unique experience in life.

    There wasn’t sudden transition from manual to automatic exchanges. The newer technology was introduced in more populated areas and less populated areas were left behind for a long time until there was enough demand to make an upgrade profitable.

    In my service area we had two “Step By Step” exchanges, the first automatic exchange technology, 3 “Crossbar” exchanges the second auto tech, many remote ARK-D and ARK-M remote exchanges that were basically the same as crossbar but just really palmed of the heavy lifting to the closest “trunk” exchange. The -D and -M stand for decadic (rotary) dialling or Mult-Frequency Code (MFC) push button dialling. I also had two manual operators in remote areas. And also 2 long path microwave links and one short path (200km) 3 channel radio link.

    I also had 2 fully electronic computer controlled exchanges. I took my then girlfriend into the middle of the second floor of one that was three storeys high and asked here where the biggest computer in [My State] was and when she couldn’t offer an answer to the question I told her she was standing in the exact centre of it.

    Anyway the whole charging system was based on the time length of call or distance as it was in the manual days. Early electro mechanical exchanges were designed on this fail charging method and at great cost to the consumer due to the burden of cost related to the extra technology required to “meter” the cost and then administer the financial transactional environment. Today, I pay AUD$30 per 28 days and nothing more irrespective of how much I phone or text. Data is different storey but out of scope.

    SO here it is simplified. A twisted balanced pair of wires went from the exchange to the phone. The twisting was to cancel crosstalk from one pair of wire (to one phone) into another pair of wires for another phone. The whole circuit from one phone to another phone was balanced (as a wired circuit) or or similarly protected from crosstalk my modulation in RF.

    Inside exchanges and between exchanges there were 4 to 6 wires.
    6 Wires were used when two exchanges were in teh same building or very close
    1 & 2) TX Voice to
    3 & 4) RX Vioce from
    5 Busy line
    6 Metering (charging for the call)

    More often 4 wires were used
    1 & 2) TX / RX voice to and from
    3) Busy line
    4) metering

    So if you want to build a simple 10or less electro mechanical exchange today then here’s the shortcuts.

    The uni-selector (switch that just goes round but not up and down) as yo have less than 11 phones you don’t need a multi-selector that steps up a number of times and around a number of times to get up to 100 circuits.

    Today you can but rotary switches that come with number of simple configurations from common parts.

    They can have –
    12 positions and one circuit
    6 positions with 2 circuits
    4 positions with 3 circuits
    3 positions with 4 circuits
    2 positions with 6 circuits

    You can probably attach a number of these end to end if you find a way to extent the rotating shaft because of the way they bolt together in levels.

    2 considerations 1) They are “make before break” unlike the “break before make” used in exchanges, you can probably design around this, if not then use two levels instead of one and alternating connections –

    level 1: 1 3 5
    level 2: 2 4 6

    So it doesn’t matter that moving from 1 to 2 shorts them together (normally) because now 1 and 2 are on different levels.

    Circuits can pass both AC and DC so you only need 2 circuits instead of 4

    So that’s 4 levels of 1 by 12 switches if you don’t want to deal with the make before break issue.

    Next actuation – solenoid ratchet lever switch shaft

    Dump the leaver and use a nuts and bolts socket set ratchet. They have 12, 24 or 48 positions. too easy.

    48 Volts leaves the exchange (via a relay to detect when you pick up the phone and another to detect dial (a 555 would be better).

    It drops over the line, by 50 KM it’s down to about 20 volts and if it get too much lower than that the old carbon mics don’t work well. So your not such with starting with 48V.

    Ring current is 60V and 60Hz would be OK so a Mains transformer down to about 40VAC-60VAC to ring the bell and put that 40VAC-60VAC through a second transformer to get it down to 3 to 6 volts (test it yourself) to male the ring “tone”.

    Old phone exchange relays had a lot of tricks up their selves.

    Each held three rows of about 12 contact giving 36 contacts of, Make (NO), Break (NC), make before break changeover, or break before make changeover – don’t bother with this, it’s too hard with how relays are made today.

    Timing (this is part of how it counted dial pulses so it’s not relay needed for what’s described here.

    To make a relay operate fast, make a very small gap with a thin sheet of plastic between the armature (actuator) and pole piece (end of the coil).

    To make the relay operate slowly, make the plastic thicker

    To make a relay slow to release, put a reverse diode across the coil.

    To make a relay slow to operate and slow to release (this is the hold relay when your dialling) half of where the coil would be was a cylinder of copper. Instead you could wind lots of copper wire around the existing coil and solder both ends together. This creates Eddie currents that slow the operation and release of the relay.

    practically, today all of this could be done with diodes and capacitors – just a higher current LCR network.

    1. Very interesting comment. Many years ago, I worked for Nortel installing switches for Bell. At one office, Strowger switches were at one end, DMS switches at the far end with crossbars in between.

  7. When I was little, my dad and me built a house telephone (right English term?) that consisted of two ordinary telephones. The circuit consisted of a transformer for the bell (48v?), another one for 12v, a pair of relays and a few diodes. If one picked up the phone, the other one would “ring”. Was very simple, no actual dailing involved. And the diodes had to be replaced now and then. Anyway, I thought I should mention it. :)

  8. I had a job inspecting petroleum storage tanks. I got to see an old city centered telephone exchange. There was one home washing machine size box that did all of the electronic networking and communication. One part of the building still had racks of cards with SouthWestern Bell stamped on them. The cards were for the landlines and there was still a large lead acid battery pack in the basement along with a large diesel generator. They kept around a handful of older desktop PCs and laptops to access these systems that were monitored. There even some still working green monochrome screens around.

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