During this project I modified a rotary phone to be functional in the digital age and learned many things about embedded development on the way.
## Functionality The device's functionality is divided into two main groups: Controller and Phone. While the phone portion, which will utilize the speaker and microphone in the handset, is still under development, the controller is fully operational. ### Controller The phone features seven buttons. The five inner buttons serve as function keys, while the two outer buttons switch between different pages of functions. Each function can trigger one of two options: MQTT messages or USB keyboard key events. #### MQTT Message Queuing Telemetry Transport (MQTT) is a lightweight messaging protocol designed for small sensors and mobile devices. Each button on the phone is mapped to a unique topic within an MQTT database. These buttons can be configured to accept either optional or mandatory numerical input from the phone's dial. The resulting messages can be used in various applications. For instance, I’ve integrated it with Home Assistant, allowing me to toggle my lights and adjust their brightness and colour using some simple home assistant automation scripts. #### USB The microcontroller within the device has built-in USB support, enabling it to function as a virtual keyboard when connected to a computer. these USB keyboard functions operate in three different modes: - **Single Fire:** The device sends a specified keyboard key event once when the button is pressed. - **Hold:** The key remains pressed as long as the physical button is held down. - **Toggle:** The key stays pressed until the button is pressed again, effectively toggling the key state. The specific keys triggered by these modes can be customized. I’ve configured them as F13-F20 keys, as these are generally unused.
## Understanding the Atea type 1949 The first step in utilizing the rotary phone was to figure out the functionality of all the inputs and how they were connected to one another. ### The dial

The dial mechanism in rotary phones, including the Atea type 1949, operates on a fairly simple design. Typically, it uses one or two signal pins: one pin remains open whenever the dial is not in its resting position, and the other pin generates a series of pulses corresponding to the number dialled. In the attached video you can see how these pulses are generated. These pulses were the primary method of signalling to the telephone exchange, and understanding this mechanism was crucial for adapting the phone to modern technology.

### The Buttons Buttons on a rotary phone are quite rare and typically found only on certain business models. These buttons were used for calling other phones on an internal network. The five middle buttons function as standard push buttons. However, the two outer buttons are more unique—they can be locked into an active state with a firm press when the handset is off the main body. Placing the handset back resets them, they are also connected together in a way that only one of these two buttons can be active at any given time. ### The Indicators Mechanical indicators, like those found on the Type 1949, are also uncommon in most devices. This model features two mechanical indicators, each equipped with an electromagnet. When powered, the electromagnet pulls a white piece of material into view, which in turn makes them look lit up. Integrating these indicators with the ESP32 was straightforward; it only needed a resistor to limit the current. ### The Chassis While the phone's chassis wasn’t originally designed as an input method, its metal construction makes it conductive and suitable for use as a capacitive touch sensor. This feature is currently functional but has not yet been utilized. ### Horn The handset of the phone lacks a built-in mechanism to detect when it’s been picked up. I may add a custom switch for this purpose in the future.
## Assembling the Electronics The most critical component for integrating all the phone’s features was the microcontroller. I selected the ESP32-S3 due to its relatively fast processing speed, which is essential for handling audio processing during calls, and its built-in USB support. I also opted for a version with an external antenna, as the metal body of the phone would likely block Wi-Fi signals. ### Audio To enable voice calls, a Digital-to-Analog Converter (DAC) and a microphone were necessary. I successfully integrated the speaker using an I2S amplifier module, but I haven’t yet managed to get the microphone working. ### Buttons To minimize the number of pins required, I used a demultiplexer. This also gave me the opportunity to learn about working with demultiplexers and writing the associated code. ### Final components These are the components that I ended up getting: - [UM ESP32 tinyS3](https://unexpectedmaker.com/shop.html#!/TinyS3/p/577100101) - [I2S Amplifier Module](https://wiki.dfrobot.com/SKU_DFR0954_MAX98357_I2S_Amplifier_Module) (unused) - I2C Multiplexer - TCA9548A - MAX4466 Microphone Amplifier These were then soldered together on a prototype board and wired up to the phone's internals ![](https://hootsifer.com/content/image/cm0bcg7ty0000xa1rvh599yf7_low.avif)
## Building the application
## Previous iteration Back in 2021 I first brought this idea to life using a raspberry pi and an unmodified rotary phone, The dial of which was simply disconnected and plugged into the pi with crocodile clips. Using some very rudimentary typescript code it was able to send actions to home assistant via its HTTP API