I've got an older vehicle that is wired for a CD changer. I'm using a few components to trick it into playing Bluetooth.


  • MCP2025
    • Transceiver for car's LIN bus
    • Regulates voltage (>12V to 5V)
  • M18 Bluetooth
    • Receive Bluetooth audio & send to vehicle's input
    • Send control commands to bluetooth device (my phone)
  • Arduino Pro Mini
    • Process car's LIN bus data (e.g. when I press next / prev on the car's radio)
    • Send control commands to M18 (e.g. next / prev)

My problem at the moment is that the audio sounds super awful, it has a loud buzzing (it's even louder than the music.)

There are 2 grounds and 2 Vin pins on the M18. At the moment I'm not using the same ground for the device and the audio outs but it doesn't seem to have helped.

Is there like a golden rule of grounding that I'm not following, or a really obvious spot I should add a capacitor?

Here's a photo and a schematic:

Schematic of the Arduino, MCP2025, M18 Bluetooth:

enter image description here

Photo of the Arduino, MCP2025, M18 Bluetooth:

enter image description here

  • 1
    \$\begingroup\$ Have you tried powering it off batteries (3x AA or a usb power bank) and disconnecting the 12V for now? To try to narrow down the issue? \$\endgroup\$
    – Passerby
    Mar 14, 2022 at 16:41

2 Answers 2


Your photo shows that your circuit is on breadboard and not yet installed in the vehicle. This leads me to believe the cause of buzz may be in your AC/DC power supply. You need a clean 12V DC source. Try an actual car battery and see if this eliminates the buzz.

  • \$\begingroup\$ I agree, but the battery voltage is still pretty noisy. An LC power filter would help placed between the MCP chip 5V and M18's Vin, and the arduino connected directly to MCP's 5V (it doesnt need clean power). A similar issue is found in RC quadcopters, which have a noisy battery voltage from powering motors that needs to be clean for analog video transmission to look good. oscarliang.com/power-filter-video-transmitter-fpv-camera Also, try the M18 on its own, with 3 AA batteries to check its noise floor, it might just be a low quality part \$\endgroup\$
    – Miron
    Mar 13, 2022 at 15:43
  • \$\begingroup\$ I should have specified, the noise is observed when this breadboard is plugged in to the car (there is a 12V + & ground in the trunk for a CD changer that I am using for this). I will try connecting directly to the battery terminals today as well. \$\endgroup\$
    – kevin
    Mar 13, 2022 at 19:25
  • 1
    \$\begingroup\$ I noticed that the audio was only coming though the left side speakers so I cut and resoldered my RCA cables and the loud buzzing is gone — I think there is still a small amount of interference that will require a filter though (I’ll try your solution @Miron) \$\endgroup\$
    – kevin
    Mar 14, 2022 at 20:56

You are reinventing the wheel. I have added Bluetooth to my vintage Pioneer stereo and my car. Connect to any AUX input or Tape input. Sound quality is like CD. This is a link to the exact product I have.


  • \$\begingroup\$ That doesn't help OP. They are tying into controls as well as audio, and building your own circuit is a valid reason on its own. \$\endgroup\$
    – Passerby
    Mar 14, 2022 at 16:38
  • \$\begingroup\$ Not all car stereos have aux or tape inputs, therefore this may not be re-inventing the wheel. \$\endgroup\$
    – Justme
    Mar 14, 2022 at 17:31
  • \$\begingroup\$ The Bluetooth device I wrote about can easily be connected to any car radio the same way you are connecting your circuit. Cut the end of the cord and add your own connecting plugs or jacks or solder the wire directly. \$\endgroup\$ Mar 14, 2022 at 20:29
  • \$\begingroup\$ @AndreyTheGreat The Bluetooth chip I'm using was about $2 and could easily be soldered directly to my radio, but the somewhat unique ability I've added is that I can use the existing stereo / head unit in the vehicle to control (e.g. next / prev song) by hijacking the CD changer port in the trunk :) \$\endgroup\$
    – kevin
    Mar 15, 2022 at 20:51

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