HZ-B500-MB VER2.1 mainboard in CarPlay/MP5 devices: firmware, pinout, and compatibility

User question

Hello Forum, I have a cheapie mp5 CarPlay unit with the HZ-B500-MB board. In the setup of the unit it has the ability to select where the driver is seated allowing the built in microphone to pick up voice when answering the telephone. It’s a very poor pickup. I wish to install an external microphone near the driver. Is this possible to achieve? If so, where on the board do I attach the wires? Thanks. I hope this is the correct forum to ask this question. Gary Roser, Sydney, Australia

Artificial Intelligence Response

Direct answer to the question

Yes, very likely it is possible.
On this class of low-cost CarPlay/MP5 units, the usual method is to remove or bypass the onboard microphone and connect an external 2-wire electret microphone to the same two PCB pads. However, I cannot responsibly tell you the exact pads on your specific HZ-B500-MB VER2.1 board without a clear photo of the microphone area, because I did not find a public schematic or readable microphone pinout for that revision. A closely related HZ-B500-MB VER2.0 board does appear in FCC documentation for model B5306, but the public material is only high-level and does not expose a usable mic pinout. (fcc.report)

Most likely connection point:

• Best place: the two pads of the existing onboard microphone.
• Not recommended: guessing at random headers or test pads. (fcc.report)

Most likely microphone type:

• A 2-wire analog electret condenser microphone, not a USB mic and not a digital mic module. Typical electret capsules use about 2 V bias, often tolerate roughly 2–10 V, and commonly present around 2.2 kΩ output impedance. (mouser.com)

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Detailed problem analysis

Your symptom is typical of these units: the built-in mic is mounted deep inside the plastic enclosure, far from the driver, with poor acoustic exposure and poor noise isolation. Even if the DSP/software is acceptable, the mechanical/acoustic placement of the original mic is often the real limitation. Replacing it with an external mic near the visor or A-pillar usually gives a much larger improvement than changing settings. (fcc.report)

1. What you should look for on the board

Open the unit and find the onboard microphone first. On cheap car units it is usually one of these:

• Round metal can, 4–6 mm diameter → usually an analog electret capsule
• Tiny rectangular SMD package with several pads/holes → possibly a MEMS microphone, in which case the mod becomes less straightforward

For your purpose, the good case is the first one: a simple 2-pad electret capsule. Typical electret microphone circuits use:

• one pad to ground
• one pad that carries bias plus audio signal through the existing board circuitry. (mouser.com)

2. Where to attach the external microphone

If your board uses a normal electret mic, the correct attachment points are usually:

Board point	Connect external mic wire to
Existing mic ground pad	mic shield / negative
Existing mic signal+bias pad	mic inner conductor / positive

That means the practical mod is:

1. Locate onboard mic
2. Identify its two solder pads
3. Desolder the onboard mic
4. Solder two wires to those same pads
5. Route those wires to the external microphone location

This is the cleanest and most reliable method. (mouser.com)

3. How to identify which pad is which

Use a multimeter before soldering the external mic:

• Ground pad: continuity to chassis ground, USB shell, or board ground plane
• Signal/bias pad: with unit powered, typically shows a small DC voltage relative to ground, often around 1.5–3 V in these circuits

That matches standard electret microphone biasing practice. For example, common electret microphone datasheets specify 2 V nominal operation, 2.2 kΩ output impedance, and only a small current draw. (mouser.com)

4. External mic type to use

Use a car external electret microphone, preferably:

• 2-wire electret type
• omnidirectional
• shielded lead or compatible with shielded extension cable
• intended for aftermarket head units / Bluetooth hands-free kits

Do not use:

• USB microphones
• PC headset microphones unless you know their wiring
• powered lavaliers with internal electronics
• digital MEMS breakout boards

The board is expecting an analog electret capsule, not a digital audio source. (mouser.com)

5. Should you leave the internal mic connected?

My recommendation: no.
Remove or disconnect the original onboard microphone and use only the external one.

Reason:

• the internal mic keeps picking up cabin noise from inside the dashboard
• parallel microphones can upset biasing and degrade signal quality
• the acoustic benefit of the mod comes from moving the pickup point near the driver

In some circuits, paralleling two electret capsules may still “work,” but it is not the preferred engineering solution. Using only the external mic is cleaner and more predictable. (mouser.com)

6. Wiring method I recommend

Preferred wiring:

• center conductor → mic signal/bias pad
• shield/braid → mic ground pad

Cable type:

• thin shielded audio cable
• keep it reasonably short
• route it away from 12 V power leads, DC-DC converters, screen backlight wiring, and USB power leads

This matters because the microphone node is high impedance and low level, so it is easy to contaminate with alternator noise, switching noise, or RF pickup. (mouser.com)

7. How to verify the board is not using a digital microphone

Stop and reassess if the onboard mic is:

• extremely small SMD package
• more than two electrical connections
• connected by multiple traces to a processor or Bluetooth/Wi‑Fi IC

If so, it may be MEMS/digital, and then a direct swap with a 2-wire electret mic will not work correctly. In that case I would need a close-up board photo before advising further. The public documentation I found for the related board family does not provide enough mic-circuit detail to distinguish this for your exact revision. (fcc.report)

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Current information and trends

I was able to verify that a closely related product family exists in recent FCC documentation:

• Model B5306 is documented as a Carplay product
• Hardware version listed is HZ-B500-MB VER2.0
• Public internal photos exist, but they are not detailed enough to give a readable microphone pinout
• I did not find a public schematic or pad map for HZ-B500-MB VER2.1 in the material reviewed. (fcc.report)

Industry-wise, the common field fix for poor call quality in budget head units remains:

• replacing the buried internal microphone
• relocating the microphone physically closer to the driver
• using shielded wiring and better acoustic placement. (mouser.com)

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Supporting explanations and details

Think of the onboard electret microphone as a tiny sensor that needs two things from the board:

1. DC bias power
2. a path to send back a small AC audio signal

That is why one pad is not simply “audio+” in the same way as a speaker output. It is usually bias + signal on the same conductor, with the audio extracted by the following circuitry. This is standard electret practice and is shown in typical electret measurement/application diagrams. (mouser.com)

A useful analogy: the microphone line is more like a sensor node than a robust audio output. That is why:

• cable shielding matters
• solder quality matters
• wrong polarity usually causes no operation or very poor operation. (mouser.com)

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Ethical and legal aspects

• Modifying the unit can void warranty.
• In a vehicle, careless wiring can create short circuits, intermittent faults, or distraction risks if the cable is poorly routed.
• Do not place the mic where it interferes with airbags, steering, or visibility.
• Disconnect vehicle power before opening the unit or soldering on the board. (fcc.report)

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Practical guidelines

Recommended procedure

1. Remove the unit from the car.
2. Open the case and locate the onboard microphone.
3. Photograph the board before touching anything.
4. Check with a multimeter:
   • one pad to ground continuity
   • the other pad for small DC bias when powered
5. Desolder the onboard mic.
6. Solder a shielded 2-core lead to those pads.
7. Connect to an external 2-wire electret car microphone.
8. Mount the external mic near:
   • sun visor
   • top of A-pillar
   • overhead console area
9. Make a test call with engine off, then engine on, then while driving. (mouser.com)

Best practices

• Use flux and a fine-tip iron
• add strain relief with hot glue or a tie-down
• keep mic wiring away from noisy power wiring
• if noise remains, add a ferrite clip to the cable near the head unit
• if the external mic sounds muffled, check whether the capsule opening is obstructed by tape or trim. (mouser.com)

Likely challenges

• Mic pads may be small and easy to lift
• cheap external mics may have nonstandard polarity
• some “3.5 mm car mics” are wired differently depending on vendor
• if the board uses MEMS instead of electret, the simple swap will fail. (fcc.report)

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Possible disclaimers or additional notes

• I can say the modification is probably feasible, but I cannot give the exact pad location from the information provided alone. The available public board-family material is not detailed enough for that. (fcc.report)
• If your exact board revision differs significantly from the FCC-documented sibling, the mic area may have moved or changed. That is an inference, but it is a reasonable one given the documented existence of VER2.0 and your stated VER2.1. (fcc.report)
• If you send a sharp, close photo of both sides of the board, especially the area behind the front-panel mic hole, I can usually identify the correct solder points.

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Suggestions for further research

If you want to continue safely, the next best steps are:

• post a macro photo of the microphone component and nearby parts
• include a photo of both board sides
• measure and report:
  • voltage on each mic pad to ground
  • continuity from each pad to chassis ground
• note whether the existing mic is:
  • round metal can
  • or tiny rectangular SMD part

With that, I can tell you much more confidently:

• whether it is electret or MEMS
• which pad is ground
• which pad is signal/bias
• whether a simple external mic transplant will work. (mouser.com)

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Brief summary

• Yes, an external microphone mod is probably possible.
• The most likely method is to replace the onboard mic with an external 2-wire electret mic connected to the original mic pads. (mouser.com)
• I did not find a public HZ-B500-MB VER2.1 mic pinout, and the related public documents for the board family are not detailed enough to name the exact pads from text alone. (fcc.report)
• Therefore, the correct next move is: open it, find the onboard mic, and work from those two pads.
• If you upload a good board photo, I can mark the likely solder points for you.