Strix Wireless headsets have no way of reporting if the battery is going to die (except the blue LED on the side) so I decided to build my own solution for battery monitoring.
It's based on an nRF51822 which is an ARM based BLE enabled MCU. This advertises the battery voltage, mic status and charging status every ~3 seconds.
The battery voltage is measued using a 680k/300k voltage divider which scales the 3.0V - 4.2V LiPo range to the 0.0V - 1.3V ADC range.
The data is put into the manufacturer field of the advertisement data and follows this format:
| Offset | Length | Description |
|---|---|---|
| 0x00 | 0x01 | Length and version data: 0b XXXX YYYY, where X is the length of this structure, Y is the version (1). |
| 0x01 | 0x01 | Flags: 0b 0000 00XY, where X is the charge LED status and Y is the MIC mute status. |
| 0x02 | 0x02 | Battery voltage ADC measurement. |
| 0x04 | 0x02 | Battery voltage (divide by 1000 to get the actual voltage). |
Most of the required connections are nicely labelled on the PCB.
| Pad on PCB | MCU Pin | Usage |
|---|---|---|
| GND | GND | Main ground. |
| VDDIO | VCC | Main 3.3V, present when the headphones is turned on. |
| VBAT | P0.01 | Battery measurement pad, connect through the 680k/300k voltage divider. The ADC is 1.3V max! VBAT is disabled when the headphones is turned off, this saves some battery life. |
| Pin 3 of SW3 | P0.02 | Mic mute detect. Pin 2 of SW3 is GND, this can be used to detect the state of the switch. |
| R1 (or pin 2 of LED1) | P0.30 | Charge detect. |
Unmodified PCB:
Connections:
Modification in place:
I decided to build a custom BLE receiver stick instead of using the BLE API from Windows, because it only works with UWP apps and most of the BLE sticks that I had did not work with this API.
The stick uses a CH340 based USB to serial converter and an nRF51822 based WT51822-S4AT BLE module. The serial converter module provides 3.3V to the BLE module which communicates using a simple 115200 baud serial port.
My goal was to create a simple taskbar icon which shows the battery and the microphone state of the headphones. This app connects to the receiver stick using the virtual serial port.
The stick accepts 2 commands:
| Command | Parameters | Description |
|---|---|---|
| 0xAF | Client MAC | Starts listening to BLE advertisements from the specified MAC. |
| 0xBF | - | Stops listening |
When a BLE advertisement packet arrives from the specified MAC the following packet is sent back from the stick:
| Offset | Length | Description |
|---|---|---|
| 0x00 | 0x01 | Data header - always 0xA5 |
| 0x01 | 0x01 | Data length (n) |
| 0x02 | n | BLE advertisement data which contains the custom manufacturer specific data specified in the headset modification section. |
Based on this data the tray icon can show the following states:
| State | Tray icon | Description |
|---|---|---|
| Unknown |  |
If no advertisement is received for more than 30seconds an unknown state is shown. |
| Battery depleted |  |
|
| Normal |  |
|
| Charging |  |
|
| Muted |  |
When the mic is muted a battery state icon and a mute icon is swapped every second. |