#rotary-dial — Public Fediverse posts

Still YUkon

My 212 number is YUkon 2. The exchange was retired as a spoken name sometime in the 1960s, when the phone company finished converting the system from alphanumeric to pure digits, and the YU that used to stand at the front of every Upper West Side number became a 9 and an 8 on a rotary dial. The number remained the same. What changed was the meaning. YUkon 2-8888 was an address. 982-8888 is a string of digits.

I have been a 212 snob since 1988, which is to say since graduate school at Columbia when a 212 number was something you earned by moving into Manhattan and something you lost when you moved out. The area code was geography enforced by the phone company. My first 212 was 529-3939 in Alphabet City, and I lost it when we moved, the way everyone lost their number in the years before portability, and the number went back to Bell Atlantic and reappeared years later at the switchboard of a Manhattan hotelier. I have the receipts. I have the memory. The 3939 was mine for the years it was mine and then it was someone else’s, and this was the deal.

Local Number Portability for wireless went into effect in 2003, a change that felt at the time like civic liberation. The FCC had decided that a phone number belonged to the person rather than the carrier, first for landlines in the 1990s and then for wireless in 2003, which meant you could take your number with you when you changed providers. By 2011 it was possible to buy a 212 number from a reseller like 212AreaCode.com and have it ported to Google Voice within twenty-four hours. I did this. I am on my eigth 212 number now and I am not apologizing for any of them. The 212 I carry today lives inside a Google Voice account that rings a phone that might be anywhere. The area code no longer tells the truth about where I sit.

The question is whether the 212 still means anything after the geography has been severed, and the answer is that it means a different thing than it used to. A 212 in 1988 meant an address on a switchboard in Manhattan. The 212 today means a claim on a city that the claimant may or may not live in. My 212 is a claim. A twenty-five-year-old in Topeka who bought a 212 from an eBay seller last week has also made a claim. We are not making the same claim, and the difference matters, but both claims are legitimate under the rules the FCC wrote.

What the 212 still signals, for those who can read the signal, is a citizen of the old city. A 212 in 2026 is an archive badge. It documents three things: knowledge of what 212 used to mean, enough care to obtain or preserve the number, and some relationship with the old city in memory or aspiration. Manhattan residence is not one of them. This is the same kind of signal as knowing which subway line runs express on weekends, or which deli closed in 2004, or when the 9 train stopped running. It is urban memory encoded in a data field. The data field still exists even after the referent has moved.

The alphanumeric exchanges were the original form of this signal. BUtterfield 8 in John O’Hara’s novel meant the Upper East Side. Pennsylvania 6-5000 in the Glenn Miller song was the Hotel Pennsylvania on Seventh Avenue, a number that answered for eighty years before the hotel was demolished. YUkon was an Upper West Side exchange. SCHuyler was another. MUrray Hill sat east of Fifth in the 30s. TRafalgar covered another slice of the West Side. A person reading a phone number in 1958 knew roughly which neighborhood the phone sat in. The area code was not used for local dialing because the exchange name already told you the neighborhood. When the area code system was built out in the 1950s and 1960s, 212 was the entire city. When 718 was introduced in 1984 for the outer boroughs, 212 gradually narrowed to Manhattan. The 212 became the Manhattan stamp at the exact moment the exchange names were fading out. One signal system replaced another.

The replacement was cleaner but carried less information. YUkon 2 told you a neighborhood. Manhattan 212 told you a borough. Portable 212 tells you nothing about location. Each step was a loss of resolution, and each step happened for good operational reasons, and the cumulative effect is a phone number that now communicates almost nothing about where the person answering it is standing. The number retains symbolic weight because a few generations of New Yorkers still carry the memory of what the digits used to mean. The weight is inherited. Inheritance is not proof of residence.

The rest of the phone number system has decayed around the area code question. Caller ID is no longer reliable because spoofing tools let robocallers display any number they want. The consumer answer to this has been to stop answering the phone. Unknown numbers go to voicemail. Known numbers from a business go to voicemail. Calls from numbers the phone does not recognize are presumed fraudulent. The phone number as a communication channel has been gutted by its own abuse, and the younger generation has responded by moving to text, to app messages, to Slack channels and Signal groups and WhatsApp threads, to every channel except the voice call. The phone number persists as a login credential and as a verification token. Its original function as a way to speak with another person has become residual.

The collapse makes the 212 a more interesting sign than it was in 1988. The 212 is no longer competitive with other identity markers because the whole identity-marker system built around phone numbers has collapsed. What remains of the phone number is the symbolic residue, and the 212 carries more symbolic residue than any other area code in the country. It retains the weight of old New York, old Manhattan, the city of pay phones and directories and the operator who connected your call. A 212 in 2026 is a period piece worn on purpose. The person wearing it is saying something about what they remember or what they want to belong to.

Nothing in this argument defends snobbery. Snobbery requires that the marker confer real status, and a 212 no longer confers real status because the 646 holder and the 917 holder and the 332 holder and the 929 holder and the 347 holder all live in the same city you do. The snob position requires a hierarchy the portability rules dismantled. What the 212 confers now is continuity. The holder of a 212 is continuing a line. That holder may have inherited the number from a parent who moved into the city in 1971, or bought the number from a reseller last Tuesday, or held the number through six moves across three boroughs because portability made it possible. The line carries the meaning, and the resolution of the signal is a footnote.

My current 212 number ends in 8888. I bought it from David Day in 2013 after searching for a number with the right weight to the ear. Before me, 982-8888 answered at the Avenue A Bistro Cafe at 103 Avenue A, at A1 Fitness Equipment Corp on East 7th Street, at Davis Design Co on East 12th Street, and at a G2 Sushi place at the same Avenue A address as the bistro. All four sat within ten blocks of the apartment where I had lived in the late 1980s with the 3939. The digits had an East Village biography before they became mine.

The 8888 was a deliberate choice. I had already learned the hard way what certain digits mean. In 2005 I had a cell number with four 4s in it, ending in 4040, and every time I called my favorite Chinese restaurant in the East Village the elderly man on the phone said “Lucky Lucky Number!” in a smoke-raspy voice when I gave him the digits. The teenage delivery driver repeated the phrase three times at the door, smiling and nodding. I thought for years I had been blessed. A commenter eventually explained that the four in Chinese sounds like the word for death, that my number was therefore dialing death by his count six times in a single phone number, and that the triple “Lucky Lucky Number!” at the door was a protective counter-chant to balance the unlucky energy I was bringing to their shop. I killed the 4040 number in 2006 and replaced it with a randomly assigned cellular number whose digits summed to a figure divisible by three, which is good in this system. By 2013 I had learned enough to select the 8888 deliberately. The number eight in Chinese numerology associates with prosperity and the doubled pair with joy, and four eights stacked at the end of a 212 is the kind of aspiration an East Village veteran can carry on a line without having to apologize at the delivery door. My ten digits carry geographic memory and cultural memory at once, and two decades of sushi orders and fitness equipment deliveries are laminated into the number I answer to today.

My YUkon 2 still answers. The exchange name is not printed anywhere on my phone, not legible in any caller ID window, not remembered by anyone who calls me who is under the age of seventy. The YU is still in the digits. Anyone who knows that 98 spells YU on a rotary dial can read the old exchange under the new number, the way you can read a previous tenant’s wallpaper under the paint in a renovated apartment. The 212 means the same thing. The city underneath is still there. You just need to know what you are looking at.

RE: https://mastodon.social/@keyboards/116244165862660996

It's been a while since I've seen a #keyboard with some fun and color like this. It's refreshing! 💜💛

#MechanicalKeyboard #purple #yellow #rotarydial #fun 🎉

Adapting an Old Rotary Dial for Digital Applications https://hackaday.com/2025/06/13/adapting-an-old-rotary-dial-for-digital-applications/ #telephonedial #classichacks #MedicalHacks #PhoneHacks #rotarydial #hardware #PCBHacks

Adapting an Old Rotary Dial for Digital Applications - Today in old school nostalgia our tipster [Clint Jay] wrote in to let us know abou... - https://hackaday.com/2025/06/13/adapting-an-old-rotary-dial-for-digital-applications/ #telephonedial #classichacks #medicalhacks #phonehacks #rotarydial #hardware #pcbhacks

What does Linux Need? A Dial! https://hackaday.com/2025/05/30/what-does-linux-need-a-dial/ #PeripheralsHacks #linuxkernel #LinuxHacks #rotarydial #dialphone

What does Linux Need? A Dial! - It’s fair to say that there can’t be many developers who have found the need for a... - https://hackaday.com/2025/05/30/what-does-linux-need-a-dial/ #peripheralshacks #linuxkernel #linuxhacks #rotarydial #dialphone

☎️ Ah, the future of web design: a site updated through a phone call, because why stick with a keyboard when there's a perfectly good rotary dial? 🎉 Spongebob's birthday wish hotline is the pinnacle of digital sophistication. Next up: sending emails with carrier pigeons! 📞🐦
https://715-999-7483.com/ #webdesign #futureoftech #rotarydial #spongebob #digitalinnovation #quirkyideas #HackerNews #ngated

Hacking a Rotary Phone https://hackaday.com/2025/03/13/hacking-a-rotary-phone/ #PhoneHacks #rotarydial #dialphone

Hacking a Rotary Phone - [Yaymukund] made an interesting observation. Old-style rotary phones were made to ... - https://hackaday.com/2025/03/13/hacking-a-rotary-phone/ #phonehacks #rotarydial #dialphone

I will take "Indestructible" for $500 Alex.... Like a family heirloom, we still have the telephone that could survive an nuclear holocaust, the Western Electric 500 Model telephone. It still works BTW and I have a sudden urge to call POPCORN #vintage #phone #rotarydial #popcorn #bell #model500 #oldschool

My diary has reminded me that in 1989, when I was living in #Albany during the week and #Syracuse on the weekends, I would have to go to a payphone down the block from my Albany apartment to check the messages on our Syracuse answering machine – because the Albany phone was a rotary dial and couldn't send the tones used to check messages.

In case you kids wondered what it was like when dinosaurs walked the earth.

#telephone #RotaryDial #RotaryPhone

A New Toy and an Old Prop https://photo.m-j-s.net/blog/2009/04/a-new-toy-and-an-old-prop/

#blackandwhite #bw #EN #fotografie #OldTelephone #photoblog #photography #RotaryDial #W48

[On someone wanting a #rotaryDial sound effect or screen on their #smartPhone]

Justine Haupt actually built a #cellPhone with a physical rotary dial! It’s an open-source design. She sold a kit with most of the parts, and is taking preorders for a new, improved version (with physical bell!).

https://www.justine-haupt.com/rotarycellphone/
https://skysedge.com/unsmartphones/RUSP/index.html

1/2
#oldtwitterposts

Rotary dial as USB keyboard:

https://petrovs.info/post/2023-01-12-shaiba/

#usb #hardware #keyboard #RotaryDial

Dial-a-#SID: The #party #jukebox for #chiptunes

https://www.youtube.com/watch?v=W5sFOk52hTI&ab_channel=lftkryo

#C64 #Commodore64 #CBM #Commodore #SIDTunes #C64Music #ElectronicMusic #8Bit #RetroComputing #Computing #Telephone #Telephones #RotaryDial #RoptaryDialling

Hackaday Prize 2022: Vintagephone Links the Past to the Present (and Future) - Brrrrrrrring! Movies and TV are one thing, but the siren song of a rotary phone ri... - https://hackaday.com/2022/05/11/hackaday-prize-2022-vintagephone-links-the-past-to-the-present-and-future/ #2022hackadayprize #thehackadayprize #voiceassistant #rotarydial

Vintage Rotary Phone MIDI Controller – Part 7

Having built my adaptor into a small box, I’ve gone back to add a mode button so I can include all the different applications into a single sketch.

This builds on the previous parts in this series.

• Part 1 – Understanding the telephone hardware and interfacing to an Arduino.
• Part 2 – Decoding the rotary dial from the Arduino.
• Part 3 – Rotary phone MIDI note controller.
• Part 4 – Rotary phone MIDI program change.
• Part 5 – Rotary phone MIDI random note sequencer.
• Part 6 – Rotary phone to MIDI adaptor.
• Part 7 – Rotary phone multi-mode applications.

Warning! I strongly recommend using old or second hand equipment for your experiments.  I am not responsible for any damage to expensive instruments!

If you are new to Arduino, see the Getting Started pages.

Parts list

• Arduino 5V Pro Mini (as I’m soldering it in, I’m using a cheap clone)
• “GPO” or “BT” original 746 rotary telephone
• Scrap ADSL filter
• 120kΩ resistor
• 2x 220Ω resistors
• Chassis mount 5-pin DIN socket
• RJ11 telephone cable (cut in half) – this is the “handset” cable or possibly a “modem” cable
• USB cable (cut in half)
• MIDI sound module

The Circuit

The idea is to simply add a push-button switch to the build from part 6, so I’m taking that as my starting point.  I’m adding a button between D3 and GND, using it in INPUT_PULLUP mode.

I made a hole in the case and made a small cut-out in the PCB to hold the button relatively firmly in place, then connected it as shown below.

Aside: I somehow managed to break the leg off the resistor between 5V and the MIDI socket, but didn’t notice until I tried it and nothing worked – so that was a bit of a pain, as I needed to gently prize everything apart enough to get a new resistor soldered in place!

The Code

So, the main idea is to use the button to change between the various modes of the phone.  I’ve implemented the following:

• Mode 1 – Send MIDI Control Change 16 – General Purpose Controller 1 – with the digit as the value (0 to 9).
• Mode 2 – Play NoteOn messages for C major, starting with MIDI note 60 (C4) for 0.
• Mode 3 – Play NoteOn messages for a pentatonic scale, as described in the “simple mode” from part 3.
• Mode 4 – Play NoteOn messages for several pentatonic scales, as described in the “complex mode” from part 3.
• Mode 5 – Send MIDI Program Change messages as described in part 4.

Note that I haven’t implemented the sequencer as described in part 5 as the code structure is quite different to the other modes.

Also note, if you examine the code, the modes are encoded as 0 to 4 within the code itself.

The main logic is as follows:

Loop:IF button pressed THEN  switch to the next modeIF there is a new digit read THEN  IF in Control Change mode THEN    Send MIDI General Purpose Controller 1 CC Message with value "digit"  ELSE IF in Program Change mode THEN    Update the program change number  ELSE    Check which scale we're using AND    Send the associated MIDI NoteOn message for that digit in that scaleIF phone is placed back on the hook THEN   IF in Control Change mode THEN nothing to do   ELSE IF in Program Change mode THEN      Send the MIDI Program Change message   ELSE     Send a MIDI NoteOff message for all notes from all scale modes, for all digits

Once again I’ve implemented it so that all notes sustain until the phone is placed back “on hook” when I simply cycle through all notes in all scales sending NoteOff messages.  This is a little redundant but does mean that if the mode had changed after notes started playing, then all notes will eventually be turned off properly.

The last thing I’ve added was some indication of which mode we’re in.  When the button is pressed and the mode changed, it will play a few notes from the C major scale starting on C5 to indicate the number of the mode (1 to 5).

Find it on GitHub here.

Closing Thoughts

I’m still wondering if it would be better to have specific MIDI messages defined for the different events within the phone, and I’m part way there with the use of the first general purpose CC message.  It might be useful to also have a CC message defined for on-hook and off-hook too.

I’ve also wondered if I should have a mode that plays proper NoteOn/NoteOff messages when dialing.  I might still implement that for the C major scale mode – I’m still deciding!  I also use the built-in LED to indicate a button press, but I might update it to flash to indicate the mode too, again I’m not sure yet.

The MIDI channel is still hard-coded to 1, and to be honest that is fine for me for now.  I would like to find a way to get the sequencer functionality in there somehow, but need to think about that a little more. I was also wondering if I should generate MIDI real-time clock messages in response to the pulses, but again I’m still chewing that over too.  I have plenty of IO pins on the Arduino spare, but not a lot of space in the casing.

I have managed to pick up a second phone and on simple inspection it appears to be wired up the same as my original phone – but for some reason it isn’t reliably detecting pulses, so I need to look at that at some point too.

So there are still plenty of things I could do next!

Kevin

#arduino #arduinoProMini #controlChange #midi #midiController #programChange #rotaryDial #telephone

Rotary Phone MIDI Controller Still Makes Calls - [Kevin] has long wanted to do something musical with a vintage rotary phone and an... - https://hackaday.com/2022/03/15/rotary-phone-midi-controller-still-makes-calls/ #arduinohacks #musicalhacks #rotaryphone #rotarydial #arduino #midi

Vintage Rotary Phone MIDI Controller

I’ve wanted to do something musical with a vintage rotary telephone for a while now, so when I happened to come across one in a charity shop, I jumped at the chance to have a go!

Be warned: there are many “vintage styled” modern phones with a pile of modern electronics inside.  This is not one of those – this is a genuine original phone.  You can easily tell if you get inside it – the electronics definitely look like something from the 70s and 80s.

On getting it home, I discovered that this one had been properly converted to be (still) usable on the UK phone system, so rather than my original plan of gutting it and adding an Arduino (like the majority of tutorials and projects you’ll find on the Internet), I wanted to find a way to use it without changing any of the circuitry or wiring inside.  This means trying to find a way to use it from the plug-in phone socket only.

This it the first in a series of posts looking at how best to do that.  In this post I describe the workings of the phone and what I plan to do with it.  If you are only interested in the final result – hooking up the Arduino and reading the dials, you may wish to skip ahead to part 2, although you’ll need to know how to wire it up as described later in this post first.

• Part 1 – Understanding the telephone hardware and interfacing to an Arduino.
• Part 2 – Decoding the rotary dial from the Arduino.
• Part 3 – Rotary phone MIDI note controller.
• Part 4 – Rotary phone MIDI program change.
• Part 5 – Rotary phone MIDI random note sequencer.
• Part 6 – Rotary phone to MIDI adaptor.
• Part 7 – Rotary phone multi-mode applications.

Warning! I strongly recommend using old or second hand equipment for your experiments.  I am not responsible for any damage to expensive instruments!

Extra Caution: You must NOT connect anything you’ve changed using any information in this or subsequent posts, or even the linked references (as I cannot vouch for their accuracy), to the public switched telephone network.

The information in these posts assumes a phone that was used in the UK. I don’t know if any of it translate across to another country’s systems.

If you are new to Arduino, see the Getting Started pages.

Parts list

• Arduino Uno
• “GPO” or “BT” original 746 rotary telephone
• Optional: Scrap ADSL filter
• 120kΩ resistor
• Breadboard and jumper wires

Background: Inside a 746 Vintage Telephone

There is a fair bit of information around about how these phones work, but nothing that I could find that talked about the actual “on the wire” protocol in terms of how you could use it with a microcontroller.

Here are some resources for reading about the technical detail of these phones:

• British Telephones – Telephone No 746 and 8746
• Sam Hallas Collection – Let’s Take a Telephone to Bits – Telephone No 746
• Steve Glennie-Smith – Rotary Dial Telephones and Broadband
• Vintage Telephone Wiring Schematics

The original circuit is described in the “British Telecom Note” N846.  You can find scans of “issue C” of this note all around the Internet, but this describes the “original state” of the wiring before the UK telecoms system switched over to “plug and socket” wiring.  The official upgrade to a 746 phone consists of the following (as far as I can tell):

• Reconfiguring some of the jumpers between the terminal connections (the T1 to T19 screw connections on the circuit board).
• The reconfigured jumpers I believe will (among other things) isolate C1 and C2 (as this functionality is now meant to be in the master socket).
• Ensuring the ringing circuit has a 4000Ω resistance, which means either swapping out the ringer circuit coils or adding a 3k3 resistor in series with the original coils.

This is the circuit from the N746 datasheet updated to use “modern” wiring (this is taken from the Vintage Telephone Wiring Schematics site – reproduced here for information – there is no implied statement of accuracy of correctness here – use at your own risk and certainly not for connecting to the public switched telephone network!).

This seems to be exactly the arrangement in my phone.  More details can be found on the British Telephones site, although I repeat that any unofficial modification like that described does NOT allow you to connect it to the public switched telephone infrastructure.

Given all this, on seeing that the wiring inside my phone has been officially updated, that the ringer unit was upgraded to proper dual-2000Ω coils, that it has a “Tele 8746 G; B.T.Q.C” sticker on it, and given at some point it also had a pre 01 UK area code and a 5-digit phone number (it was written on the reverse of the label), leads me to believe I have a proper, old, but usable-today unit.  And it does indeed ring and allow me to make and take calls, so my exchange must still support pulse-dialing (at least until 2025 anyway).

So, whilst there are a number of tutorials on the Internet about how people have rewired phones and installed Arduinos inside them, or taken out the rotary dial unit and used that directly, I was determined to see if I could find a way to use my phone “from the outside” without any changes inside at all.

The key is understanding a little how the four wires in the BT style “flat” socket are used.  Here are the resources I’ve found:

• British Telephones – How does PST work?
• Steve Glennie-Smith – Telephone connectors, wiring and colours

Of the six possible conductors in the flat plugs, only four have a meaning, and often only two or three are actually used:

• Pin 2 – “Line B” – Red
• Pin 3 – “Bell” – Blue
• Pin 4 – “Earth” – Green
• Pin 5 – “Line A” – White

The availability of a “bell” signal is really from the days when there was a separate link required to make the phones actually ring.  The “Earth” I think is only used when connected to a PABX, so goes unconnected in my phone.  These days, I believe there tends to be just two conductors to the “outside world” (the “A” and “B” lines) with the “bell” signal achieved via a modulation of the Line signals creating a third signal from the electronics within the phone master socket (via a “ring capacitor” I believe).

Pairing all this information with the previous circuit diagrams and the photo of my own phone, then it looks like I can maybe use the RED wire as an INPUT to the Arduino with the WHITE wire to GND and then the whole phone will act as a switch-to-ground when in use…

But the only way to really find out is to have a go!

BT Flat Phone Plug to Arduino

I could have done any of the following to gain access to the RED and WHITE terminals:

• Remove the original lead and hook directly to the relevant screw terminals inside the phone (RED goes to T8 and WHITE goes to T18).
• Cut the original plug off and access the RED and WHITE cables directly.
• Find an old phone extension cable and cut off a BT style socket and expose the wiring that way.

In the end, I opted to use an old ADSL filter pcb as a “BT Phone Socket Breakout”, as I seem to have a fair few of these kicking around for some reason.  To do this, I did the following:

1. Removed all components apart from the sockets from the PCB.
2. Identified the four connectors from the BT plug.
3. Soldered on the required cables. In my case I started with RED, WHITE, and BLUE, as I wasn’t sure if I’d need BLUE or not.  It turns out RED and WHITE are sufficient.

That first photo shows an Arduino Pro Micro for scale.  As you can see there are some interesting possibilities there – but let’s not get ahead of things for now…

The test circuit for now is therefore as follows.  Note: this gets a minor update – see later!

The Code – Part 1

At this stage, I’ve gone for total simplicity to try to see what is going on.  Here is the code I’ve used:

void setup() {
   pinMode(2, INPUT_PULLUP);
   Serial.begin(9600);
}

void loop() {
   Serial.println(digitalRead(2));
}

This literally just samples the IO pin that is connected to the RED wire and prints the value out, which will either be HIGH or LOW – i.e. 1 or 0, to the serial port.  This allows me to fire up the Serial Plotter and have a look at the signals directly.

Here is a sample trace, when dialing number 7.

There are seven distinct pulses and a bit of noise.  This already has the phone off the hook.  When the phone is on the hook, the input reads a constant HIGH all the time.

This starts to give a hint at how the pulses might be decoded:

• Look for the long HIGH going LOW, indicating the phone is now off the hook.
• Count the sequence of pulses that follow for the first digit.
• Do the same to subsequent digits, noting the longer gap between the last pulse from one digit and the first pulse for the next.
• Notice when the signal goes HIGH for a long time again, signifying the phone is now back on the hook.

But exactly how best to do this will be the topic for the future.

Arduino and Telephone Tests

Whilst I don’t really understand the telephone’s circuit, I thought it would be prudent to take a few measurements to see if that makes it clearer what is going on.  Here are the current and voltage measurements across the WHITE and RED wires at various stages when connected to the Arduino:

• On Hook: Constant 4.5V at ~9uA.
• Off Hook: Constant 2.25V at ~80-85uA.
• Dialing: Pulled round prior to release: 0V at ~155uA.
• Dialing: Released: Pulses go from 0V (~155uA) to 4.5V (~9uA).

I don’t quite understand that “off-hook” half-voltage measurement, so this is my simplification of the Arduino/Phone circuit to help trying to think about what is going on.

My primary initial concern was the current load on the Arduino… but actually those values should be fine.  The ATmega328 datasheet suggests a maximum current of 20mA for an IO pin (40mA for short periods at a push) and 100mA for VCC and GND.

The “on hook” switch is a normally open switch (open when “on hook”, closed when “off hook”) and basically just disconnects the WHITE cable when the phone is on hook.  This means the IO pin will simply read HIGH.  I’m not sure why there is a current measured across the RED wire (9uA) as I believe the only current should be through the Arduino’s internal PULLUP resistor from VCC via “leakage” into the IO pin.  The ATmega328 suggests a maximum current leakage of 1uA, but I wouldn’t be able to measure that from “outside” anyway I think?  So I’m probably missing something here, but given the levels involved I’m not too worried for now.

When “off hook” the switch is now closed and that completes the circuit.  RED is now connected to WHITE (GND) via the phone’s circuit, including passing through the MIC.  I’m guessing that the combined resistance of the phone circuit (through the MIC) is comparable to the resistance of the Arduino’s internal PULLUP resistor, so the circuit is acting like a potential divider – hence reading half-voltage at the IO pin.  The current reading on the RED wire is around 80-85uA, which presumably is a current from VCC to GND, so that is well below the Arduino’s limits.

When the dial is first turned, but not released, then the normally open “dial” switch is closed which shorts out the MIC. In this case, the voltage now reads 0V, and there is now a current measured on the RED wire of 155uA.  If this is now flowing from VCC via the telephone’s circuit (skipping the MIC) through to GND.

When the dial is pulsing, it is opening and closing the “dial pulse” switch, so is making and  breaking the circuit between the 0V/155uA state (with RED connected to WHITE skipping the MIC) and the 4.5V/9uA state (the telephone circuit is disconnected, so it is just the internal PULLUP reading HIGH).

My experiments seem to indicate that the “off hook” state can be read successfully by finding a HIGH to LOW transition, but with the resulting voltage of 2.25V feels a lot more by luck than judgement.  The ATmega328 datasheet describes the LOW state as being less than 0.3 x VCC and HIGH as being more than 0.6 x VCC.  Typically values of <1.5V and >3V are used for a 5V system, so this is directly within the “undefined” region.

If the phone circuit and the pull-up are indeed acting as a potential divider, then there are two possibilities:

• Reduce the resistance of the phone circuit.
• Increase the resistance of the PULLUP resistor.

The simplest option on the phone circuit is to mimic what happens when the dial starts – it shorts out the MIC which removes the MIC’s resistance from the circuit.  But if I’m trying not to change the telephone itself, then I might not want to do that.  It also permanently disables the MIC.  Another possibility then is to use an external PULLUP resistor instead.

The ATmega328 datasheet is a little vague on the value of the internal pull-up, but it says it is guaranteed to be between 20kΩ and 50kΩ so rather than try to calculate a value, I tried a few out to see what happens.

The compromise here is that as the resistance is increased, then the voltage for HIGH is reduced as well as the voltage for LOW.  It cannot be reduced below the minimum for a HIGH reading – i.e. 3V.  It will be acting as a potential divider with the INPUT pin’s impedance (well technically it won’t that is too simplistic as I understand it, but there will have an interaction and that will do for now).

There are several calculations suggested in various tutorials for calculating pull-ups but I’ve not really found any that were particularly useful…

In the end, I just tried a few resistors and made some measurements and found the following.

So it looks like an external pull-up of 120k gives me a HIGH reading of >3.5V and a LOW reading of <1.75V.  That still isn’t perfect – ideally it would be <1.5V – but it is a lot better than 2.25V!

This therefore leads to the following updated circuit and code.

And the new code just uses INPUT mode rather than INPUT_PULLUP.

void setup() {
   pinMode(2, INPUT);
   Serial.begin(9600);
}

void loop() {
   Serial.println(digitalRead(2));
}

The results are largely the same, but I believe the off-hook indication will be a lot more reliable when I get that far.

In fact, I actually went one step further and updated my BT plug breakout to include the pull-up resistor, so now it simply has three connections to the Arduino:

• 5V (the BLUE wire – I just reused what I had)
• GND (WHITE)
• Signal (RED)

So here you can see it going “off hook” then dialing three.

Closing Thoughts

This is a great start.  I was concerned that I’d have to compromise the internal wiring somehow in order to get a signal out from the phone, but it looks like I won’t have to do that at all now as the on/off hook and dialing pulses are plainly visible to the Arduino.

I did try to work through the circuitry to work out how applying a signal might work “through” the phone, but in the end gave up and just starting having a go.

There may still be a better option at some point, and I don’t know what those components are making of the signal from the Arduino’s side – they are used to a -48V or -50V signal on the RED cable for example, but I think I get an appreciation at the top level of what is going on.  I suspect most of that circuitry relates to the “AC” side of the phone – the part involving the MIC and ear-piece.

Kevin

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