Analog transistor radio upgrade project

This project transforms a vintage German transistor radio — the SABA Mainau — into a modern smart audio device powered by Raspberry Pi 4 and Raspberry Pi Pico. While preserving the iconic design and analog audio amplifier of the original, the project adds support for Spotify Connect, Internet Radio, and a TFT display interface, fully controlled by physical buttons and a rotary tuning dial.

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📦 Hardware Overview

• Raspberry Pi 4 — main processing unit (audio, web interface, display)
• Raspberry Pi Pico — microcontroller for reading physical I/O (buttons, tuning dial)
• 1.77" TFT SPI Display (128x160, ST7735)
• Original SABA transistor amplifier — reused as final stage for audio output

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🔧 Hardware Integration Plan

• Remove original radio circuits, leaving only the transistor audio amplifier.
• Mix stereo output from Raspberry Pi’s 3.5mm jack into mono using two resistors. And inject this signal into radio's circuit.
• Mount TFT display to the front panel, connected to SPI interface of the Pi 4.
• Connect physical buttons to Raspberry Pi Pico.
• Use the original variable capacitor (from radio oscillator) as a tuning dial:
  • Read its analog value via RC circuit and ADC input on the Pico.
• Connect Pico and Pi 4 via UART: Pico acts as an I/O controller, sending input events to the Pi.

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🎯 Functional Features

The device will support two modes, switchable via physical controls:

1. Spotify Connect Mode

   • Acts as a Spotify Connect device via Raspotify.
   • Use physical button for Play/Pause.
   • Use tuning dial to skip to Next/Previous Track.
   • TFT screen shows track info and album art via Spotify API.

2. Internet Radio Mode

   • Streams MP3 radio stations via VLC.
   • Use physical controls to Play/Pause and switch stations.
   • TFT screen displays station name and status.

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🧠 Software Architecture

• OS: Raspberry Pi OS Lite
• Audio:
  • Spotify: Raspotify (daemon)
  • Radio: libVLC (called via .NET)
• Main Application: .NET 9 WebAPI hosted on Raspberry Pi 4
  • BackgroundWorker for input handling and mode management
  • SQLite database to store:
    • Radio stream URLs
    • Spotify authorization tokens
• GPIO/SPI/UART: Accessed via PIGPIO C library + .NET interop (P/Invoke)
• Frontend UI:
  • Web interface built with Angular
  • Used for initial setup and Spotify authentication
  • Hosted together with WebAPI app, accessible in local network via NGINX

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🔗 Communication Between Components

Component	Interface	Role
Raspberry Pi 4	UART	Master, receives input events
Raspberry Pi Pico	UART + ADC	I/O controller (buttons, dial)
Pi 4 ↔ TFT LCD	SPI	Display track/station info

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🧾 Documentation

All schematics, wiring diagrams, and specifications are in the Docs/ folder:

• Raspberry Pi Setup instructions
• System Architecture
• TFT Display And PICO Wiring
• Pico I/O & RC Tuning Circuit
• SABA Upgrade Circuit Plan

Diagrams are stored in Docs/Img/ and created using draw.io.

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🗂 Repository Structure

Folder	Description
RadioApp	.NET WebAPI and main worker service project
RadioFrontend	Angular "admin" application for Spotify authentication and setup radio channels
RadioIO	C firmware for Raspberry Pi Pico
Docs	Markdown documentation & diagrams