Electronics hobbyist: installing bluetooth in an old car radio?

I like to do handicrafts in my spare time and came across the following problem. I have an old Ford Fiesta with a cassette radio. Now I would like to have my music played on my mobile phone and I thought about upgrading the existing radio with Bluetooth. I have already seen one or the other video on Youtube for this, but it doesn't really want to work. That's why I'll describe my problem here: I removed a bluetooth module from an old headphone that can also be operated with usb, so I assume: 5V operating voltage is okay this module has soldering points for aux right / left, as well as their respective GND's. My car radio also connects the cassette player via audio right / left and a GND for both. Now I have soldered the respective places accordingly. Now to my problem: If I operate the Bluetooth module with an external power source (e.g. Laptop) it plays the sound without any problems. However, as soon as I tap the power source from the radio myself (I soldered a USB cable to it with a voltage regulator that regulates it down to 5V) it no longer delivers any sound. With both options I can run a cassette at the same time. With the first option, it automatically takes the Bluetooth source and plays the music from there. With option 2, the cassette even goes silent as soon as I connect the GND wire of the Bluetooth module to the GND of the sound chip of the radio.

If not clearly explained, please just ask and please do not make pointless comments like "Just buy a bluetooth-enabled radio" "There are cassette adapters for that" etc. I want to tinker and I want to get it done. It apparently works as long as it takes an external power source.

Perhaps the 5 V source is too weak or it is driven by a power supply that does not provide this additional load. The USB 2.0 Spec states 0.5 A, which is 2.5W.

look at the measuring device o, it still brings more than 4.75V when connected.

Otherwise, make a pencil sketch and take a photo, a professional can better imagine something underneath than with 10 sentences of description.

Post the answer to another comment just below:

"That's how I did it. See my explanation" However, as soon as I tap the power source from the radio myself (I soldered a USB cable to it with a voltage regulator that regulates it down to 5V), it no longer delivers any sound. "The bluetooth module gets Power. Only when I connect the GND connection of the aux connections to the radio circuit board does the sound go out. For me, the electrical circuits are more of a problem. As soon as I operate the module with the power source of the car radio and do not connect it to the car boxes cheap headphone shells, it works. "

Apparently you connect the GND incorrectly.

Disconnect the GND connection at the inputs of the radio and leave it with a connection to the GND of the radio.

Your Bluetooth audio output (for headphones) could be a bridge amplifier.

With a bridge amplifier, loudspeaker (+) and loudspeaker (-) are both supplied with signals in opposite directions. I.e. Your supposed GND also has no fixed ground potential.

Here is an example of a bridge amplifier, then you will understand better:

https://www.mikrocontroller.net/topic/430570

My proposed solution for you:

You have to filter out the DC voltage components.

Use a coupling capacitor (~ 100nF) for each L + R to decouple the audio signals from the Bluetooth amplifier.

And only connect the GND line to one side of the radio, only connect R and L of the Bluetooth audio output to the radio amplifier input. In other words, the ground of the audio lines is only connected on one side in the radio, the other side of the ground line is not soldered to the Bluetooth circuit board.

This separates the DC voltage components from the Bluetooth board and the car radio board. And you can then wire your fixed voltage regulator as planned, the GND potential will then implicitly reach the radio via this.

Coupling capacitors? If they are too small, you will lack the bass (depending on the impedance). Then simply use larger coupling capacitors (bipolar ELKOs ~ 10uF).

Coupling capacitors purpose can be seen here:

https://www.elektronik-kompendium.de/sites/slt/0204302.htm

PS:

Without a proper power adjustment (Ri = Ra), there could be distortions or overdrive, the sound is then bad.

Which IC (no, designation) is on the bluetooth board?

Radio model?

On which IC (sound chip? TDA? O. Similar) do you have d. Audio line soldered on?

which PIN's used?

Thank you very much for your detailed answer, then I will have to go further. Nevertheless, I would like to briefly show what combinations I have tried and what result came out, because I still can't figure out why it works with an external power source and not with the radio power source.

First combination:

Power source for bluetooth module: laptop

Audio connections: Audio L / R / GND to the audio chip of the radio

Result: the sound is played

Second combination:

Power source for bluetooth module: car radio

Audio connections: Audio L / R / GND to the audio chip of the radio

Result: No sound is played. As soon as audio L / R from the Bluetooth module are connected to the audio chip, there's a beeping or noise like when a cell phone is held too close to the speakers.

Third combination:

Power source for bluetooth module: car radio

Audio connections: Audio L / R / GND to external speaker shells

Result: the sound is played

I have too little understanding of the subject and don't understand why it doesn't work when the radio's power source and audio output (in this case the radio's audio chip) are no longer working together in one circuit. As long as these are separate, the power source and audio output will work separately.

I use the same pins that the cassette tape uses. I couldn't find a pinout map of the chip. The Bluetooth module is CSR 8635 b04U

I'm a radio electronics technician and can explain your situation again.

The PIN assignment of the CSR8635 is explained a bit here:

https://www.electrodragon.com/w/CSR8635

Pay close attention to the 2 paragraphs in the link

"Headset Connection" (headphones)

"Differential four wires" (Speaker 4 Ohm / 3Watt)

The situation is as follows:

The CSR8635 has (as I suspected) a bridge amplifier (see differential output) so the 4 lines of the speaker outputs can't simply be interconnected somehow.

Here are the relevant PINs:

19 CHARGE External Battery Charge Management (5V)
20 VBAT power input (3.3 ~ 4.2V)
21 1V8 1.8V output
22 GND power ground
…
36 SPK_RN audio right channel negative differential output terminal
37 SPK_RP audio right channel positive differential output terminal

38 SPK_LN audio left channel negative differential output terminal
39 SPK_LP audio left channel positive differential output terminal

Here again my suggestion:

At first only connect the supply voltage from the fixed voltage regulator +5 V and GND (that's already running, as you write).

The chip has 4 outputs for headsets or speakers.

You are only allowed to use the positive outputs (37 SPK_RP and 39 SPK_LP) (not PIN 36 nor PIN 38) by connecting these 2 outputs each with a coupling capacitor (100nF = 0.1uF) in series to your radio amplifier (? Input L and input R). The GND input remains unconnected on the radio.

Finished! It should work!

PS:

I repeat again. If the fit isn't right, you have poor distorted sound! And if you do not connect the shielded NF cable to GND on one side, you will get interference noises from the ignition system when the engine is running into the radio when you hear via "Bluetooth".

100nF = 0.1uF capacitors (bigger is also possible, no problem) https://www.reichelt.de/folienkondensator-100nf-250v-rm5-mks2-250-100n-p172708.html

As an electronics technician, you often have to fly around somewhere.

Many thanks for your effort. I'll try to implement that. I just couldn't understand why it worked with one power source and not with the other.

You just have to understand my statement, otherwise it won't work.

With a bridge amplifier, loudspeaker (+) and loudspeaker (-) are both supplied with signals in opposite directions. I.e. Your supposed GND also has no fixed ground potential.

Addendum:

The interconnection of PIN 36 and PIN 38 to a GND connection is not permitted with the CRS8635. See shading of Headset Connection (two coupling capacitors 220uF are also used there) …

Quote from my link:

https://www.electrodragon.com/w/CSR8635

"Headset Connection"

SPK_LN and SPK_RN connect either one, not both

…

I will help you to understand:

Your radio is supplied with 12 V (+) and (-). The (-) is connected to the sheet metal housing (GND) of the entire radio.
ADDITIONALLY, on your cassette input (+/- right channel and +/- left channel) both (-) connections (GND) are also connected to the sheet metal housing of the radio. You can also measure it with a continuity tester. The GND cassette is connected to the GND supply voltage.

That now means that everything you connect to these two GND points will be connected to each other via the sheet metal housing. For the CRS8635 this means that you switch PINs 36 and 38 to GND. That means a short circuit of the two outputs PINs 36 and 38 to GND, which is not good, the CRS8635 chip can break.