Using a microphone with the RN52 Bluetooth Adapter


A request from a customer for advice surrounding our BAL (RN52 Development Kit) product, based on the RN52 Bluetooth Adapter from Microchip, wanting to use the product for streaming audio from a microphone to a separate Bluetooth master recording device using the HFP protocol led to more headaches than we could have expected. 

The Microchip data sheet didn't provide us with much help, other than to confirm that our electret microphone (found here and bought here should you want to try this yourself) was within the recommended specification. We looked to this article by sixerdoodle (whom we have to thank for setting us off in the correct direction) and tried to build a circuit for our microphone connection. 

mic and RN52 BAL board


Gaaaah! No luck. Excuse the mess in the photo, we were hoping for a quick fix, not something we'd have to write about. We found, similar to the blog linked above, that the MIC would sometimes work on the first call, and after that either be ridiculously quiet, or not work at all. Unideal for a customer to have to turn it on and off every time they start recording. 

Then it stopped working altogether, even on first power up.

The MIC_BIAS pin was not producing anywhere near enough voltage or power, it was (very inconsistently) producing 1.4V open circuit and dropping to 0.5V when in use. Not enough for a 3V mic.


Back to the drawing board

The RN52 is based on a CSR BlueCore5 chip similar to this one. Now the CSR chip is much more advanced than the RN52 as Microchip chose not to implement all of its features. We read here that the MIC_BIAS pin derives its power from BAT_P, a pin used by CSR as part of a battery charging circuit for bluetooth products. This is not a feature which Microchip has implemented in any capacity, and so we put our voltage level issues down to this. We found the circuit shown in one of their datasheets, suggesting the MIC_BIAS was to be used as an enable pin for a regulator to provide power from the mysterious BAT_P pin. We couldnt get our MIC_BIAS to drive anything... a Darlington pair, an L.E.D, nothing. 

CSR Circuit


We took a 3.3V rail from elsewhere on our project board and tried to implement the circuit like above that with the RC values shown on the datasheet with the diagram. Still, the voltage drop across the microphone was not high enough. Significant playing around, and the addition of a capacitor across the mic finally yielded what we had worked two days to achieve. A phone call where we could actually converse!

See to the left our final design, which now works just fine! C1 (100uF) is a capacitor that removed the background hum on the input, and by increasing that, it removes it gradually. If the 100uF doesnt work for you try change it to 220uF and it should remove the issue.

A lot of effort for a mono phone call, no?

BAL Launch – We are nearly there

Photo original

Hi everyone,

Today we are submitting our kickstarter campaign for aproval for the new BAL – Bluetooth audio link. As you can see from the previous posts, it has been a long time in development, but we are nearly there! We have a brand new board capable of bluetooth audio streaming through an amplifier and pop filter so you can use it in any high end sound system. Not only that but we have broken out all the pins on the RN52, as well as making sure you can get into the inner workings of the chip so you can control the controlled device from your computer or any micro controller with a serial out function. If the BAL is connected to a phone, you can also use it to make and answer phonecalls.

The video for our board is here: Video

To find out much more about the BAL find all the documentation on the BAL menu link above.

Please have a look at our campaign, the link is: KickStarter

And finally, thank you to all the people on Reddit who are helping us out before the launch! Please join in the conversation – Reddit

Jacob Rawson

RTC Using a SeedBoard

Hi everyone,

If you haven’t heard, me and my co-partner are building a development board to embed into projects rather than using a full blown Arduino. For more details see this post.

Anyway, as one of the first projects for this little board, I have hooked it up to a DS1307 real time clock, and an AXE133Y OLED display to make a simple clock. As shown below:

SeedBoard Rev 2.0 with RTC


I have simply connected the I²C lines to the same pins on the Arduino and wired up the ds1307 like the data sheet tells you to do. If someone asks I will draw up a schematic for you all, though many can be found online – like this one.

I hope to do many more projects in the coming weeks and months prior to the launch of this board next year so keep posted to see what else the SeedBoard can do. And as always you will be able to find the links to the Arduino sketches on our github page linked at the top of this page.

Thanks guys,


SeedBoardSeedBoard with RTC