My goal is to build a bluetooth hands free device, basically a speaker phone, to pair with a mobile phone. I am using the Roving Networks RN-52 Bluetooth Audio module mounted on a breakout board from Sparkfun (product). Connecting a speaker and configuring the device for Hands Free Profile (HFP) has been no problem. However, I am having difficulty connecting a microphone. I have a electret condenser mic and breakout board, also from Sparkfun (product).

The RN-52 has five audio input related pins: Mic_R+, Mic_R-, Mic_L+, Mic_L-, and M_bias. The electric mic has three pins: Audio, Ground and VCC.

I have tried connecting the audio and ground of the mic to both the + and - of left and right mic inputs on the RN-52 with no luck. (VCC is connected to 3.3v).

I am certain that the mic connection is my problem, as the module and speaker work fine for streaming audio over A2DP.

I would sincerely appreciate any help or advice.

  • \$\begingroup\$ I wonder if the following is related, maybe you need some sort of firmware update (if it's been released yet): forum.sparkfun.com/viewtopic.php?f=13&t=36736 \$\endgroup\$
    – PeterJ
    Commented Feb 16, 2014 at 5:26
  • \$\begingroup\$ Thanks for the tip @PeterJ. That firmware update concerns master and slave modes. My RN-52 is in slave mode, however Hands Free and Headset Profiles allows a slave device to send audio to the master device. \$\endgroup\$
    – Joel37266
    Commented Feb 17, 2014 at 20:28

1 Answer 1


The datasheet contains microphone connection for regular electret microphone (middle of the schematics on p.13 -- this involves 2k2 bias resistor to MIC_BIAS, and input on MIC_L-/MIC_R- via 47nF capacitor).

However in your case, the microphone already contains amplifier. So I advice connecting microphone in non-differential way: MIC_L- goes to ground, MIC_L+ goes to microphone output via small capacitor (any value of 47nF or more will work). Make sure to use 'lime mode' and not 'microphone mode' in your setup.


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