There are people who throw away old telephones - and then there are those who find them in the garbage and think, „How can a microcontroller actually read the digits from a rotary dial?“ This talk follows the journey of transforming a classic German FeTAp 611 rotary phone into a mobile device while keeping its vintage charm. Building on earlier retrofits, this project aims to combine the following design goals into a mobile version of the Fernsprechtischapparat:

• Grandparents-compatible – The phone shall be easy to use by non-technical people, showing the same look and feel as the original phones, including details such as a dial tone.
• easy phone switching – Switching between FeTAp and regular cellphone shall not require unscrewing the phone to switch SIM cards.
• standard components – PCB/PCBA suppliers shall be capable of manufacturing boards at a reasonable price.
• device-agnostic circuit design – Adapting to different phones (e.g. W48, FeTAp 791, FeTAp 611) shall minimize the need for changes in the schematic. This includes a ringing voltage generator that shall be powerful enough to drive an old W48 phone.

This talk will walk you through certain aspects of the German analog telephony standard 1TR110-1, and the challenges faced when implementing those on a battery-powered device with little space. It explains

• the state machine implemented on an STM32 microcontroller,
• how to connect old carbon microphones to modern audio electronics,
• designing (and avoiding mistakes in) a flyback based SMPS to generate 32V - 75V ringing voltage,
• how to generate 25 Hz AC using an H-bridge,
• and how to layout the PCB such that the ancient second handset connector can now be used for USB-C charging.

In the course of the development, I discovered that the project is not only a good way to get a glimpse into various aspects of ancient and modern types of electronics - but also into people’s reactions when such a phone suddenly starts ringing on a flea market… :-)