Bluetooth Intergration Into 1980's Blaupunkt Radio

I have added Bluetooth and handsfree calling functionality to my 1980's Blaupunkt car radio while keeping as much original functionality as possible.

This radio is from my 1989 Mercedes 190E and I wanted to keep a period radio while also not needing a portable speaker to listen to my music. I temporarily used a cassette to aux converter but the audio quality was terrible therefore needed a more permanent solution.

This was achieved by hijacking the button that originally would have cycled between FM, AM and LW (Long Wavelength). This button now toggles between FM and Bluetooth. The cassette player functionality has also been retained. From my research leading up to starting this project, it seemed most people sacrificed cassette functionality to insert Bluetooth audio.

The core principle of how I got this to work revolves around a 3 pin mechanical lever switch on the main PCB that is lifted when a cassette is inserted into the unit. When this lever is raised, the radio audio is cut and audio input is taken from the cassette reader head instead. I used this principle to “trick” the unit into thinking a cassette had been inserted when it hadn’t, allowing me to play Bluetooth audio, while also allowing the insertion of a cassette to still cut out either FM or Bluetooth audio and play as usual.

Once this principle has been established it was just a case of some simple if statements of Arduino code, a few transistor gates and an audio gate chip designed to block or allow the flow of an unamplified audio signal controlled by a high or low signal.

This is less of a “how to do it” and more of a “what I did” instructible. I hope it’s still of interest and makes sense.

1. Arduino Nano
2. Perf Board
3. Bluetooth to L&R with Aux Mic Module
4. 16 Resistors
5. 14 x 10k
6. 1 x 5.1k
7. 1 x 220
8. 3 x NPN 2N222A Transistors
9. 2 x P-Channel IRF9540 Mosfets
10. 1 x M74HC4066B1, STM, Quad Bilateral Switch
11. Soldering Iron
12. Solder
13. Wire (22 AWG max required)

First step was to open up the radio and try and find the left and right audio traces. I used a service manual to trace back from the output downstream to before the internal amplifier. This is where I decided to inject the Bluetooth audio. I also started probing the lever switch to determine how it behaves and hence how to manipulate it to play Bluetooth.

Next step was to set up a breadboard prototype to test out the Arduino code

Once the prototype was proved to work, I designed a perf board design on KiCAD. Space was at a premium, so everything had to fit on one perf board. The perf board and Bluetooth module just about fit inside the unit. I traced down where I needed to solder contacts for all the inputs and outputs on the PCB. The traces for the FM/AM/LW button had to be disconnected to allow them to be repurposed. A cradle was also 3D printed to support and insulate the perf board and Bluetooth module.

The final thing to do was to fix the female aux mic. Luckily there was a spare blowout hole at the back of the case used for radio antennas. I 3D printed a grommet and interference fitted the mic aux in place.

The microphone then needed to be installed in the car. I utilised a button blank in the centre of the dashboard and 3D printed a replacement with a cradle for the mic.

And with that, the radio is done! The FM/AM/LW button now toggled between FM and Bluetooth and the cassette player still functions as normal. The whole process took about 6 months of on and off work.

I'll attach the data sheets, code and KiCad files. There is inevitably a more efficient way at doing this but the whole thing only cost me about £30 and was a fun project.