Interface a Rotary Phone Dial to an Arduino





Introduction: Interface a Rotary Phone Dial to an Arduino

An old rotary phone can be used for a number of purposes in your Arduino projects - use it as a novel input device, or use the Arduino to interface a rotary phone to your computer.

This is a very basic guide describing how to interface the dial to an Arduino, and get the number dialed passed into a computer over the Arduino's serial link.

Step 1: Remove the Dial From the Phone

First step is to remove the dial unit from the phone. I'm using a GPO phone of some sort from the 1970s.

On this phone, the dial popped straight out - I just needed to give it a tug. If it doesn't, you may have to open up the phone and work out how to get it off.

There were five cables connected to the back of the dial unit. On my phone, these were regular spade connections, so I loosened the screws and pulled them out. If you want to re-assemble your phone, remember to record which color wire goes to which connection.

Step 2: Identify the Switch

Once the dial is out, it should be relatively easy to see how the dial converts rotary movement into pulses. Try spinning the dial by hand and watching the movement on the back. You should see a switch making and breaking a circuit rapidly - so if you dial '9', the switch should engage nine times.

For those of you who may never have used a rotary dial before - remember that the dialing only happens when you let go the number and let it spool back .

I've documented how it works for my phone in the Notes of the photo below.

There's also a blurry video of the mechanism working.

Step 3: Make the Circuit

Once you have found the switch that is being made and broken, you should be able to identify the connections by following the wires back to the connection terminals. In my case, the two sides of the switch are connected to the two leftmost terminals.

Hook up these terminals to some jumper wires, and get prototyping! The switch in my dial is always-on, and is broken for each pulse when dialling, so I used the very simple circuit below. Pin 2 will go HIGH for each pulse as the dial rotates.

When the phone isn't being dialed, the switch in the dial unit is closed (a so-called NORMALLY CLOSED switch, for obvious reasons) so the circuit connects pin 2 to ground (which to the Arduino is LOW). This is because there is much less resistance through the 470 ohm resistor than the 10K resistor.

When the phone is being dialed, the switch opens and closes rapidly (for a 9, it will open and close again nine times, remember). When the switch is open, pin 2 is not connected to ground - instead it is connected to the 5V supply through a resistance of 10470 ohms. This is interpreted by the Arduino as a HIGH.

If your dial has a NORMALLY OPEN switch, then swapping the positions of the 10K resistor and the dial should do the trick.

Step 4: Develop the Code

Now we need some code for the Arduino to count the pulses and send the total number per number dialed back through the serial port.

My code's below. As we're dealing with mechanicals here, yours may differ. Try playing about with the debounce constant and the 'how long do we wait before assuming dial has finished rotating' constant.

I've tried to comment it as neatly as I can. Hopefully it's pretty simple.

int needToPrint = 0;
int count;
int in = 2;
int lastState = LOW;
int trueState = LOW;
long lastStateChangeTime = 0;
int cleared = 0;

// constants

int dialHasFinishedRotatingAfterMs = 100;
int debounceDelay = 10;

void setup()
pinMode(in, INPUT);

void loop()
int reading = digitalRead(in);

if ((millis() - lastStateChangeTime) > dialHasFinishedRotatingAfterMs) {
// the dial isn't being dialed, or has just finished being dialed.
if (needToPrint) {
// if it's only just finished being dialed, we need to send the number down the serial
// line and reset the count. We mod the count by 10 because '0' will send 10 pulses.
Serial.print(count % 10, DEC);
needToPrint = 0;
count = 0;
cleared = 0;

if (reading != lastState) {
lastStateChangeTime = millis();
if ((millis() - lastStateChangeTime) > debounceDelay) {
// debounce - this happens once it's stablized
if (reading != trueState) {
// this means that the switch has either just gone from closed->open or vice versa.
trueState = reading;
if (trueState == HIGH) {
// increment the count of pulses if it's gone high.
needToPrint = 1; // we'll need to print this number (once the dial has finished rotating)
lastState = reading;

Step 5: Check It Works!

Check it works by opening up a serial window (I use screen on a unix machine, you may want to use Hyperterm or similar on Windows), and try dialing some numbers.

Make sure that the serial program is set to read from the USB->serial adaptor in your Arduino (check the Tools->Serial Port menu in the Arduino software if you forget what that is), and a baud rate of 9600 bps.

You should see the correct number pop up as it's dialed.

Step 6: Hook It Into Something Useful!

I came up with a Quartz Composer file on my Mac to take the input and render it nicely to the screen. Once it's in the machine as serial data, you can do anything with it.

Waiting to hear your ideas!

I'll come up with a video of it 'in action' and printing the numbers to the screen as soon as I can get someone to hold the camera for me - wish I had three hands.

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thanks for posting, good coding!

I just made this work on an ancient piece of GPO test equipment originally manufactured by Plessey. This is going to be used to send commands to a project that will ultimately do great things on Nixie Tubes - thank you so much for posting the code, worked first time.

Thanks for the instructions - made it work! If I want to make the arduino to open a switch, ie making a LED light up, when a specific number is dialed, how would the code be? Any suggestions? Thx

Hey. Have you found any solution for it? Cheers, Thomas.

Do you have a sketch where this converts rotary pulses to DTMF tones? I want to use an old rotary phone generating DTMF for dialing.

If you want to do it yourself using arduino hardware, I found a german website of someone who did it, including circuit descriptions and source code:
Be aware, that you should replace the arduino bootloader and program the chip directly to be able to dial right away. The microprocessor, being powered by the telephone line, will start running only after you pick up the phone. If you keep the original arduino bootloader in, it will wait to be programmed before starting to run your own code, and this takes some time.

Just as a side note, I picked up an old old rotary phone at a yard sale and it works great on FIOS! But this would help since no one believes you when you push 0 and need help because you can't touch tone their menus.

jayne15 - With a rotary to DTMF or Touch Tone converter if you need Operator / Attendant, you would dial 0 (zero) the circuit would convert the pulses to the appropriate tones and the system would then dial thru. The same goes for "press 1 for... or Press 2 for... " the converter circuit counts the pulses and the generates the tone desired. Here is a link to a device that does just that, this is it costs about $45. You install it under the base cover of the phone Hope this helps.

Hey @JMorton3 any chance you managed to do this? I'm looking to do something fairly similar for presenting a number of tracks in an exhibition.

I actually wanted the phone to ring and then play a track when the receiver is lifted. but that might be too complex...

Hey @JMorton3 any chance you managed to do this? I'm looking to do something fairly similar for presenting a number of tracks in an exhibition.

I actually wanted the phone to ring and then play a track when the receiver is lifted. but that might be too complex...