There are a lot of options available to add an auxiliary input to you car radio, but in my opinion, they are mostly sub-par.   They either require a cable to be attached to your phone,  and we have enough cables as it is, or they use a poor sounding FM transmitter or tape input.    I was looking for a solution where my smartphone audio and phone could play directly through my car radio like the latest Tesla.    With some help from the other sites and some adjustments, the results in this post are pretty close.

In this hack, I show how I upgraded my 1997 BMW E36 M3 car radio by adding a hands-free Bluetooth receiver into the factory radio.     Now, when the tape selector is pressed, instead of a tape, the Bluetooth receiver will be played.   The hands-free microphone is mounted on the tape door so that calls and dictation from my smartphone can also be made.   In what follows,  I'll show you the steps I took so you can do this with your car radio using an inexpensive Bluetooth speaker.

2017-06-10 18.11.03 

Parts for the projects:

  • Car stereo with cassette tape player or CD player.  My model is a BMW C33 US DIN made by Alpine.
  • Bluetooth receiver module:   I hacked mine from an inexpensive Bluetooth speaker, model T3 from Roverbeats.
  • LM78M05 5v output 0.5Amp Linear Regulator
  • 2 x 10uF 25v Capacitor
  • Electrical tape

Project Steps

Step 1: Hacking a Bluetooth receiver

I cannibalized the Bluetooth receiver from an inexpensive portable Bluetooth speaker.  I purchased my Roverbeats Bluetooth speaker from Amazon for $23,   but I have also purchased similar types from China for just $7.  You can also find just Bluetooth Receiver modules on Ebay, Alibaba,  and other hobby sites, but they may require a few extra parts (i.e. A pre-Amp Stage).    Hacking a Bluetooth speaker will have almost everything you need.     Just make sure the device has a microphone and the A2DP  protocol for easy smartphone interaction.      With a screwdriver take it apart and identify the major components.

The speaker will have a Bluetooth Reciever IC or module (CSR 8635 module),    a pre-amp stage (LM358),   amplifier stage (HT6809),  microphone (mic1),   antenna (zig zag copper trace and wire) ,   Micro-USB charging port and Li-PO battery connector (J2).   For our project, we need the receiver module, the antenna, the microphone and the pre-amp stage.

To use this device for our hack, the following was done:

  1. The battery was be removed from J2
  2. A 220ohm dummy load was placed across the j2 terminals to represent a small dummy battery charging load.
  3. A 6inch twisted pair of wires were attached to the pre-amp output and will send audio to the car amplifier.
  4. The microphone was removed off the PCB and extended with a 6 inch twisted pair of wires so it can be mounted to the car amplifier tape door.
  5. A 5v linear regulator was attached to the micro- USB connector to power the module from the car amplifiers switched 12v supply.   Two 10uF capacitors were placed between Vin and Ground and Vout and Ground respectively for local bypass.
  6. A twisted pair of 24AWG wire was connected to the 5v linear regulator and ground to receive power from the car amplifier.

reworked-BluetoothSpeakerThe speaker has a battery that won't be needed in the car,   So it was removed by disconnecting it from J2.     During testing,   under light loads,  a small tone was present when the volume was turned up.     This was fixed by adding a small 220ohm dummy load in place of the battery.     Better capacitor bypassing of the LM78M05 may also help,   but the resistor was a simple fix.

Finding the output of the pre-amp to add the twisted pair took a little hunting.   The LM358 has two output pin (pin 1 and pin 7).   Using a multimeter and good eyes I traced the output of the pre-amp through a 330ohm resistor and ceramic capacitor (~10uF) to the input of the Amplifier stage (HT6809).   One lead of the twisted pair was placed after the 330ohm resistor.   The ground lead of the twisted pair was attached to the large bypass capacitor attached to the ground plane.  Zoom into the image to see the exact location.

TwistedPair ConnectionThe wires to the microphone (identified as mic1) were removed and a twisted pair of 30AWG wire were attached.    Use two 1/4 inch heat shrink tubing pieces to over the joint to eliminate the risk of a short.

I didn't have the schematic for my BMW C33 US DIN radio,    So I did not find a 5v supply that would turn off when the amplifier was turned off.   The 5v supply I did find was always present.    I wanted a switched supply so that when the car is turned off,   The Bluetooth module will also shut down.    So after some looking , I found a 12v supply that shut off when the amplifier was turned off.    A LM78M05 linear regulator was used to bypass the micro-USB connector so that this 12v supply can power the Bluetooth module.   The ground pin of the LM78M05 was soldered to the case of the Micro-USB connector since this is connected to the circuit board ground.    The Vout pin was soldered to 5v pin of the Micro-USB connector.    I found this with a multimeter by powering the speaker with a USB cable.    A 10uf capacitor was soldered also from the Vin pin and ground.    In addition, A 10uF capacitor was soldered between the Vin and Ground pins.    A 24AwG twisted pair was soldered from the Vin pin and ground pin of the LM78M05.


Step 2: Attaching the module to the Radio

After taking off the top cover, you need to remove the tape Cassette module from the unit, locate the tape audio output signals (Left Channel, Right Channel, Signal Ground) on the tape module, and locate the 12v switched power supply voltage on the main amplifier board.

bluetooth-hackdisasembledAttached in series to the "L" and "R" pins are a couple of 4.7uF capacitors.    They are used to DC isolate the tape module from the Car Amps Amplifier stage.     These need to be removed so the tape head will also not be trying to output signal.    Now,   only the Bluetooth signal can send an audio to the car amp when a tape input is selected.

TapeCassetteConnectionThe switched 12v supply was found on the power supply daughter card.   The label on the PCB silkscreen was "SW Batt".     The Ground label was also very close.    The twisted pair power leads were soldered to these power supply pins.

bluetooth-12vsupplyConnectionThe tape deck needs to be adjusted so that it thinks a tape always present.   Otherwise the tape module will not detect a tape and record an error.   This is simple.   Just remove the two springs shown and ensure the tape mechanism is adjusted so the double throw,  single pole switch is opened.    Now the tape mechanism won't move even though the tape motor may run.      The Bluetooth module can now be placed on top of the tape mechanism,   But first put electrical tape on the back of the circuit board to keep it isolated from the tape mechansim.


Step 3: Adding the External Microphone

Mounting the microphone in the table door is all that is left .    Find a drill that has a slightly larger diameter barrel than the mic package and drill into the tape door.    Then mount the mic with some tape.   I also added some foam direct behind the microphone to help direct sound even more.

Bluetooth-Tape_Microphone-LocationI hope this was helpful, good luck with your hack and let me know if you have any question.

Additional References

Here is a good reference on the web that I found helpful for this hack.