I’m a weak-willed man. I was fully intending to write this column on the topic of simulating radio frequency (RF) designs and devices, including communications and radar systems, when… SQUIRREL!!!

To set the scene and ensure we’re all tap-dancing to the same skirl of the bagpipes—as I previously mentioned in my Retro Gadgets, Gizmos, Tools, and Technologies column—I’m currently having a lot of fun blathering, jabbering, and prattling about the technologies of yesteryear in a series of Throwback Thursdays columns on Hackster.io

For example, my two most recent Throwback Thursdays offerings are Holy Sizzling Soldering Irons, Batman, in which I describe my first soldering iron (a traditional copper bit device that I had to heat over a ring on my mum’s gas cooking stove in our kitchen), and Radical Retro Cans, in which we dip our toes into the quirky history and timeless style of retro headphones.

As fate would have it, I was happily enjoying a few moments of downtime, rooting around the Projects portion of the Hackster.io website, when I ran across Brett Walach’s Retro Rotary Dial Internet Connected Lock project. “That looks like fun,” I thought. Apart from anything else, this would provide a great teaching tool to introduce and address all sorts of concepts, including switch bounce.

I know I have one of these old telephone dials lying around somewhere. I just had a quick root in the boxes filling every available space in my office, and I found a full-up phone as seen in the photo below, but I’m saving that for another project on another day.

My full-up rotary dial telephone (Source: Clive “Max” Maxfield)

This took me back to when I was a kid growing up in England in the 1960s. The phone in our house was very similar to the one shown here. It was big, it was black, and it squatted on a small table in the hallway next to our front door. In the drawer of the table was a large, thick book called a telephone directory. This contained the names, addresses, and telephone numbers of every person and business that owned a phone in our city.

Seeing this retro beauty also reminded me that one of the first electronic projects I ever built was an audio generator that produced a sound like a telephone ringing. My best friend Jeremy lived just around the corner from me. Jeremy and I had a lot of fun with this project. We started at my house. We hid the sound generator under the telephone cabinet at the bottom of the stairs. Then we ran the control wires under the edge of the carpet, snaking them all the way to the landing upstairs.

Next, we crouched behind the banister at the top of the stairs. We used the control wires to make our generator produce a repetitive “ring-ring, ring-ring, ring-ring…” Sound. We continued to make this sound until my mum came to answer the phone. We stopped as soon as she picked up the handset.

Mum looked puzzled when she realized there wasn’t anyone on the other end of the phone. She returned the handset to its cradle, muttered something to herself, and went back to whatever it was that she’d been doing. We waited for a few minutes, and then we did it all over again…

… and again…

… and again…

… and then my mum looked upstairs and realized what had been going on (there may have been some giggling involved). I bet she thought it was funny. I bet she was laughing inside. Unfortunately, she wasn’t laughing outside. She gave us a meaningful look. We knew what that look meant. It meant “It’s time for you to go to Jeremy’s house and play this trick on his mother!” (At least, we thought that’s what it meant.)

I couldn’t stop myself. I just went on eBay and purchased a vintage rotary telephone replacement dial. This is billed as being a “brand-new, unused, unopened, undamaged item.” Of course, the seller had to open the box to take the picture, but I know what he means.

If you are grizzled and gray like me, you’ll know how these things work. It seems so obvious to us that we forget that younger people—who have only ever been exposed to push-button interfaces (bless their little cotton socks)—are largely unfamiliar with these mechanisms, so a brief overview is as follows. You pick the headset up with one hand and then dial the desired number, digit by digit, with the other. To enter a digit, you insert a finger into the corresponding hole and rotate the dial clockwise until your finger reaches the finger stop. When you remove your finger, an internal spring mechanism causes the dial to return to its original position, generating a series of pulses along the way.

Do you remember Star Trek: The Original Series? I’m thinking of the time Spock is presented with an ancient civilization’s errant computer system. He manages to get it up and running in time to save the day without even having to consult the manuals. By comparison, there are multiple videos on YouTube that illustrate how baffling technology can be to anyone who is not familiar with it, like the two offerings below showing young people trying to use rotary dial telephones.

Have you ever noticed that nothing is as simple as it seems, and that there’s always more to things than meets the eye? For example, the alphanumeric annotations on the dial I just ordered are typical of a traditional North American rotary telephone, as illustrated below.

Traditional North American rotary telephone dial (Source: Dhscommtech/Wikipedia)

I must admit that there are several aspects to this that I’d not consciously noticed before. Is there anything here that catches your eye? Why don’t you take a moment to think about this before we proceed?

Yes, you’re right, the numbers start at 1, with 0 representing 10 at the end. Also, there are no letters associated with the number 1, and the letters Q and Z are nowhere to be found. It turns out that (at least in North America), the ‘1’ was reserved for special functions like long-distance dialing. The designers left it letter-free to avoid confusion and preserve it for system purposes like signaling or future expansion.

Early telephone systems used alphanumeric dialing (e.g., calling “PEnnsylvania 6-5000”), and the letters were assigned to numbers to help users remember numbers as words.

If we also exclude the number 0, which was used to call the operator (assuming it was the first and only digit you dialed), this leaves eight digits (2 through 9). Limiting the alphabet to three characters per digit (so 3 x 8 = 24 letters) simplified the design of the dials. Since Q and Z were infrequently used in names and words at the time, especially in English, the telephone engineers deemed them non-essential for telephone exchange names.

The more one delves into this, the more interesting (and confusing) it gets. In early implementations, the pulse train was generated on the forward rotation of the dial, but this proved to be problematic when users paused or varied the speed of rotation. The solution was to employ a spring to return the dial to its home position, with its speed regulated by a governor device, thereby generating a precisely timed series of pulses on the return rotation of the dial.

Also, you’d think it would have made sense for every country to adopt the same basic rotary dial design and implementation… but that would have been too easy. In North America, for example, where the numbers on a traditional dial followed the sequence 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, the number 1 was associated with one pulse, 2 was associated with two pulses, all the way up to 0, which was associated with ten pulses.

In Sweden, by comparison, where the numbers on a traditional dial followed the sequence 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, the number 0 was associated with one pulse, 1 was associated with two pulses, all the way up to 9, which was associated with ten pulses. 

Daring to be different, the folks Down Under in New Zealand decided to use ten pulses minus the number desired, so dialing the digit 7 produced three pulses. And, just for Giggles and grins, Norway adopted the North American system… except for its capital city, Oslo, which opted for New Zealand’s offering. All this makes my head hurt.

But we digress… suffice it to say that I’m really looking forward to the arrival of my new telephone dial and to coming up with some creative experiments centered around it. Speaking of which, I’d be very interested to hear any ideas regarding potential dial-based projects that pop into your mind.