I recently bought this Bluetooth audio module from Amazon (also available for a lower price on Aliexpress). The cool feature of it is that it can pass through audio. It has an aux input where another analog audio source can be connected. A relay automatically switches from the aux input to the Bluetooth chip when a Bluetooth device is connected. This is very useful in my car since it only has an aux input and I wanted to add Bluetooth to it without loosing my aux input. With this module I can connect it between the existing aux input and the radio of my car. This means I cut the cable between my car radio and the aux socket in the car. Then I connect my car radio to the audio output of the Bluetooth module and the aux input of the Bluetooth module to the aux socket in my car.

The Problem 🙄

This works fine but there is one problem I don’t like: The Bluetooth module identifies as audio sink for media (music, video…) and calls on my phone. Routing calls to the Bluetooth module is a big problem because it has no microphone, so if someone calls me, they cannot hear me. One might say “just turn of the headset function in the Bluetooth menu of your phone”. Yes, that works for one session because this setting gets lost if I reconnect to the module (and I mean just switching the power off and on. NOT repairing the device).

The Solution 🕵👨‍🔬

So, I thought: “Lets add a mic to this module. If it identifies as a headset, there must be a mic input on the chip.” Of course, there is no datasheet available for the chip therefore I had to do some hacking. Everybody who worked with audio equipment knows that you can couple hum into a high impedance audio path just by touching connections. This is what I used to find the mic pin on the chip. I called my self from another phone and listened to the audio coming from the Bluetooth module. Then I touched each pin of the Bluetooth chip with a (metal!) needle which had a direct connection to my skin. As I hoped I heard a load hum when I touched pin 12. This must be the mic input!

The Circuit 💡

At first, I tried the classical configuration with the 100nF coupling capacitor in series to the mic input and a 2k resistor to the 3V3 rail for the capsule phantom power. This worked but the audio had a loud hum mixed to it. Some audio ADCs which allow direct connections to mics have an extra voltage regulator to provide 3V to the capsule. Due to that I started to search for such a voltage output and found one. To pin 11 (directly next to the mic input, what a coincidence 😁), a cap is connected on which I measure exactly 3V. In the next step, I connected the 2k resistor directly to this cap to provide phantom power. This worked like a charm. It’s not HIFI audio but good enough for a phone conversation and noise free. See the pictures below to do the changes on your board. Additional 100pF should be connected in parallel to the mic capsule to block RF interference (when using a cable, on the side of the Bluetooth module). Pin 10 should be used as signal ground for the mic.

Additional Information ✅

Markings on my Bluetooth module:
Model: AS1711BTSE
Ver: 1.2