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.
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:
- The battery was be removed from J2
- A 220ohm dummy load was placed across the j2 terminals to represent a small dummy battery charging load.
- A 6inch twisted pair of wires were attached to the pre-amp output and will send audio to the car amplifier.
- 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.
- 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.
- A twisted pair of 24AWG wire was connected to the 5v linear regulator and ground to receive power from the car amplifier.
The 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.
The 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.
Attached 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.
The 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.
The 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.
I 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.
- Youtube video on adding an external Bluetooth module to E36: https://www.youtube.com/watch?v=BbpoVnQHanY
It seems like you can also connect to blue tooth power supply near the radio’s on off button. That circuit board near the button switch is marked GRD and PWR SW…
Hi Bruce,
Thanks a lot for the question and feedback. I recall that I also saw that signal (i.e. PWR SW) when I was doing my sleuthing, but decided it was a digital signal only to the always-on system that controlled the “Switched 12v” power. Do you have any more details on this PWR SW signal or where I could pick up a schematic? What could make the Hack simpler is to find a “Switched 5v Supply” in the amplifier, but I did not find one. This would allow the 12v Linear Regulator to be removed. If I can find a schematic, I also want to reprogram the screen so that when I select the TAPE for the hands-free/bluetooth signal, the screen will show “BLUETOOTH” or something similar instead of “TAPE”. I believe this info may be in the EEPROM IC.
Thanks
Mark
I’ve had the same idea with the same Bluetooth module but I have no intentions on using the hands-free because the microphone doesn’t seem to be very clear. With that said, I’m going to run a utility USB power supply and auxiliary female to the glove box. I can place the entire bluetooth module in the glove box without taking it apart and then plug it directly into the power supply and auxiliary. I will also be able to simply exchange the Bluetooth module if it fails. I will more likely try to locate a power supply that turns on and off with the ignition. Otherwise I will use the same power supply you have located on the circuit board. I do like your idea of hands-free better if I can find a clear microphone. Maybe then wire the microphone closer above my head area near the visor.then run that out to the glove box as well with a modify adapter to the Bluetooth module to connect the microphone. Then all I will have to do when exchanging to a new Bluetooth module is to modify the microphone with a new adapter and plug it in from the glove box. I have an old clip mic laying around the house that I may try.
Thanks for the feedback. The microphone reception on this module is surprisingly clear. I make hands-free calls with the microphone mounted in the tape door as shown all the time. However, a closer microphone would probably be better. For me, I liked the idea of having it all within the original stereo enclosurer. Your idea to adding a USB charger to the E36 Glove Box is cool. I would be interested in how you get it done as I want to add that to my car. FYI, the Glove box of the E36 already has power (i.e. to charge the Flashlight), but it does not turn off when the ignition is off.
I’ve decided to do it the way you have it modified inside of the radio. The only difference is that I am going to use the earphone and usb/aux adapter plugins instead of soldering wires to the blue tooth device except for the mic. I’ve retested the microphone and you are absolutely correct that the mic is very clear. I’ve drilled the hole in the cassette door for the mic. I’m also leaving the blue tooth device including the blue tooth battery inside the bottom portion of the casing. It fits a little snug with a small bulge of the radio lid but it fits. The ideal of modifying the led read out to say Blue Tooth instead of Tape 1 is the icing on the cake. Let me know when you’ve figured that out… I’ll be looking into it as well.
Okay… It’s done and it works perfect. I’ve use the same power line as you’ve instructed. It turns on and off the blue tooth along with the radio power button. I’ve kelp everything in the casing and used the plugins instead of solder. My hole in the door is much bigger than yours. The hole is the exact diameter of the mic. The “TAPE A” title stands out. Looking to modify it ASAP… Thank you for your guidance and feedback..!
Hi Bruce, that is great you got your system working and a good Tip for others about using the Auxilliary output from the Blue-Tooth module so you don’t have to solder the twisted pair output wire (i.e. just use the provided jack). For some reason, I thought the Aux Jack was an input. 🙂
The TAPE A / TAPE B does make our stereo’s look a little RETRO (circa 1990s). But it would be a great HACK if we could find a way to change it to BLUETOOTH. I’m looking for schematic and doing some research. Maybe this weekend.
*Note* some of the Bluetooth mini speakers do not have the headphone jack output. With the headphone connection option you don’t have to worry about the 2 capacitors and soldering. Just simply use the headphone jack.
I’ve just experimented on a few other mini Bluetooth speakers and you’re right…. The ones that say AUX are for input only…. You’ll have to locate the component that has the head phone jack symbol in order to use the connection output. It does make sense to remove the 2 compactors but in my case everything worked great with them intact. Removing the capacitors would be Plan B if all else fails.
Thanks Bruce for the updates.
Hey mark,
How would you handle a singular button? Maybe I missed it in the tutorial — my speaker had a single button that acts as power and the pairing button when held down. I want to be able to pair in case I ever get a new phone, but also want to make sure I don’t have to tap it to get power…. or would that just be by bypassing the button for power?
Thanks so much!
Thanks for the question. Most of these bluetooth speakers will go into pairing mode automatically if they don’t detect a transmitter (I.e. your phone). That is how my Bluetooth transmitter works and I have several other that work the same way. Check to see if yours also work that way.
Also, I expect that your power/pairing button is part of the battery circuit in you speaker. This can be removed and bypassed so the Bluetooth transmitter will come on when 5v is applied. Looks like some fun experiments are in order.:)
Absolutely! Thanks for commenting back. Going to remove mine this week and try some things out. I have the same stereo so luckily it should be mostly identical — I’ll be interested to see how it sounds when I’m done haha.
Is a pre amp required to do this mod? I bought two speakers yet neither have a pre-amp IC.
Depending on you car stereo input sensitivity level, I expect you may not need it. In the Bluetooth speaker, The pre-amp stage is used to provide enough signal level to drive the speakers with some power. The car stereo does not need that much power, so I bet you will be fine. Just find the wire connection that goes to the Bluetooth speaker and hook these instead to you car stereo input.
I did a simple line in hack where i tapped in to the tape feed years ago, had to use a blank tape or input adapter, but it had to be one where the reels were connected or it would ‘tape error’. I guess it’s some kind of tangle protection, but could still play tapes. Now the rubber band in the tape player snapped and I get “tape error” no matter what :(. Tried removing the springs and leaving the switch open, but no cigar. Any ideas short of repairing the tape deck?
I’m sorry I don’t have an idea. While the motor still turns in my mod, No tape is needed.
Thanks for the reply. Might be one of the differences between the C33 and C43. I ended up using a simple large, thin, rubber band, which is working fine, and ordered a cassette tape deck drive belt assortment for when that gives out. I wasn’t able to find a diagram, but the belt path for these tape mechanisms happens to be shown in the below instructable, for anyone interested:
https://www.instructables.com/DIY-Cassette-Deck-Drive-Belts/
Great work and thanks for link.