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.
Best Answer
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.