• Accurate, zero-lag audio feedback variometer using Kalman filter fusion of accelerometer and pressure sensor data. This project uses the KF4D kalman filter algorithm from the ESP32_IMU_GPS_BARO_VARIO project.
• WiFi webpage configuration with the vario acting as an access point and web server.
• WiFi Over-the-air (OTA) firmware update
• You can build a minimal audio vario with the above functionality. Or you can add optional features :
  • Bluetooth transmission of LK8EX1 sentences with a HM-11 bluetooth module. You can then use flight instrument apps like XCTrack on a smartphone/tablet with accurate barometric altitude and climb/sink rate data.
  • Push-pull L9110S driver with conventional loudspeakers for higher audio volume.
  • Torch/lantern mode using a 0.5W white led. This is accessed with a long press of a button when the unit is in vario mode. Once in lantern mode, you can cycle through 3 different brightness levels and an S.O.S. flasher mode.
• PCB design with same form factor as a single-cell 18650 power bank case.

• Ubuntu 20.04 LTS AMDx64
• Arduino IDE version 1.8.16
• External Arduino libraries used (install using Arduino Library Manager) :
  • ESPAsyncTCP
  • ESPAsyncWebServer
  • AsyncElegantOTA
• Arduino ESP8266 LittleFS Data Upload plugin

• ESP-12E/12F module (4Mbytes flash)
• CJMCU-117 10-DOF IMU module with MPU9250 and MS5611 sensors
• HM-11 Bluetooth module
• L9110S IC used as a push-pull driver for louder volume
• Kicad used for schematic and layout
  • Archived Kicad project
  • PCB layout Gerber Zip
  • Schematic PDF

EasyEDA online visualization of PCB layout.

I have not used a pcb footprint for the HM-11 module because there are several options for this module available on ebay/aliexpress with different pinouts, all called "HM-11".

1. HM-11 original. Make sure the pinout matches the one in the HM-10 datasheet (which includes info and pinout for HM-11). Example.

2. JDY-08 "HM-11" clone. This has to be reflashed with HM-10 firmware using a CC debugger adapter. If you do get a JDY-08, and have access to a CC debugger, here are instructions for reflashing it and the resulting change in pinout.

The CJMCU-117 NCS pin should be connected with a wire directly to the CJMCU-117 LDO regulator 3.3V output. The pcb already has the connection between NCS and PS (see the schematic).

The 10K I2C pullup resistors on the CJMCU-117 board should be replaced with 3.3K for a reliable interface at 400kHz.

The TLV75533 3.3V LDO regulator has a high current rating of 500mA and is suitable for the ESP8266 power supply, which has high current spikes > 350mA on wifi transmit bursts. You can use it for both LDO regulators on the board. The reason I used a cheaper, more readily available XC6219 3.3V LDO regulator for the HM-11 is because the bluetooth module current draw is low.

The optional circuit components are marked with dashes on the schematic. Do not populate them if you don't want the torch option or the bluetooth option or the L9110s loud(er) speaker option.

You can first test the board with a direct connection from AUD pin to a piezo speaker and the other piezo pin connected to ground. You will have to put solder on the jumpers to select this. If you mount the vario on your shoulder and/or have an open-face helmet, the volume level is probably enough without populating the L9110S push-pull driver circuit.

If you want louder audio, install the L9110s circuit option. If this is still not loud enough, replace the piezo transducer with a magnetic coil loudspeaker of at least 8 ohms impedance. Make sure you use at least a 47 ohm resistor for R5 to keep current pulses manageable. Ensure there is no air path from the front of the loudspeaker to back or else the front wave will cancel the back wave. Use silicone to seal the edge of the speaker to the pcb, that will do the job. Put some soft foam tape on the back of the speaker so that vibrations don't get transmitted to the MPU9250 or MS5611.

Replacement speaker-phone (NOT earpiece) drivers for mobile phones are a good choice. You can put two in series for 16ohms impedance, but make sure they are in phase.

A few components may not be readily available on Aliexpress/Ebay. You can find them on Mouser/Digikey :

• Ferrite bead 600ohms@100MHz : BLM18AG601SN1D
• TI TPS22918 high-side switch
• Broadband piezo speaker : PUI Audio AT2310TLW100R, Kingstate KPEG006
• Power switch : ALPS SSSS916400 (good quality, expensive) or SK12D07 (ebay, cheap, cut off the end lugs).
• For torch LEDs up to 0.5W, populate one of R1, R2 with a 22ohm 2512 0.5W package. For higher wattage LEDs, add a second resistor in parallel.

The ESP8266 has internal power-on bootstrap pullup/pulldown requirements for some of the GPIO pins. These cause circuit quirks on reset and during ROM boot. You will find the torch LED comes on at full brightness when you press the reset button, or put the ESP8266 in program mode.

The silkscreen markings for the ESP8266 UART are for the connecting component. RXD should be connected to the RX pin on the USB-UART module, and TXD to the TX pin.

Similarly, the silkscreen markings for the HM-11 interface are for the HM-11 module. The BTRX pad should be connected to the HM-11 RX pin. To reset the HM-11 with factory default settings, short the KEY pad to ground for a couple of seconds.

Battery current drain of ~27mA operating as audio vario with bluetooth disabled.

Battery current drain is ~37mA operating as audio vario with bluetooth LK8EX1 message transmission @ 10Hz.

If you are using an ESP8266 dev module, expect ~10mA more due to the USB-uart circuitry.

The MPU9250 MEMS chip is sensitive to mechanical stress. You can see this when you calibrate the accelerometer and gyroscope bias offsets - if you press down the module to keep it flat, you will record large offsets. For final mounting of the board module, it's best to use two-sided adhesive foam tape (no screws), this will also help in dampening external vibrations.

• Feature support compile options are in config.h
• For a minimal audio vario with the ESP8266 directly driving a piezo transducer, set CFG_BLUETOOTH, CFG_L9110S and CFG_LANTERN to false. If you want support for louder volume using the L9110S push-pull driver IC, set CFG_L9110S to true.
• To support bluetooth $LK8EX1 message transmission with a HM-11 module,set CFG_BLUETOOTH to true.
• To support the torch/lantern feature, set CFG_LANTERN to true.

• In the Arduino IDE v1.8.16, select Tools->Board->ESP8266 Boards (3.0.2) : Generic ESP8266 module
• Select Tools->CPU Frequency : 80MHz
• Select Tools->Erase Flash : All flash contents.
• Select Tools->Flash Size : 1MB (FS:None OTA:~502kB). Note : we are using a 4MB ESP-12E/F, but first selecting and flashing a different partition format fixes issues with the EEPROM flash calibration data area not being erased for the first boot.
• Press and hold the PCC button. Momentarily press the Reset button. Release the PCC button. Now the ESP8266 will be in download mode.
• Flash the application firmware. Results don't matter.
• Select Tools->Flash Size : 4MB (FS:1MB OTA:~1019kB)
• Press and hold the PCC button. Momentarily press the Reset button. Release the PCC button. Now the ESP8266 will be in download mode.
• Flash the application firmware. You should (again) see confirmation of full chip erase before flashing the firmware.
• Select Tools->Erase Flash : Only sketch option. From now on, use this option whether you are uploading the LittleFS partition image or the firmware binary.
• Press and hold the PCC button. Momentarily press the Reset button. Release the PCC button. Now the ESP8266 will be in download mode.
• Select Tools->ESP8266 LittleFS Data Upload. This will build a LittleFS flash partition with the contents of the /data directory, and then flash it to the ESP8266. The /data directory contains the static HTML and CSS files for the WiFi server webpage.
• Select Tools->Serial Monitor, make sure the unit is resting horizontally on a table. For accelerometer calibration, the MPU9250 sensor needs to be horizontal with the x and y axes flat and the z axis pointing vertically.
• Press the reset button. This is first boot and there is no calibration data. You will see a checksum error message, followed by automatic accelerometer and gyroscope calibration. You will not have to do anything if the unit is resting horizontally. Accelerometer calibration needs to be done only once, even if you flash updated firmware / LittleFS data, as long as the firmware update has not changed the calibration flash data structure.
• The gyroscope is re-calibrated each time on power-up due to issues with drift and environmental conditions. When you hear the count-down beeps for gyroscope calibration, ensure the vario is un-disturbed, in any orientation. If the vario is disturbed during the gyro calibration process (e.g. you turn it on while in flight), it will use the last saved gyro calibration parameters.
• This is a build and startup serial monitor log. You should see similar results.
• If you are using an ESP8266 development board with circuitry for automatic program and reset (e.g. NodeMCU), the board will automatically restart after loading the firmware and set zeros for the calibration values. So you won't get the same serial monitor logs as I did. But it will still work, as the zero values are detected as 'uncalibrated' and you will see a message to prepare the vario for accelerometer calibration.

To put the vario into WiFi AP server mode, switch on the vario and immediately press and hold the PCC button. When you hear a confirmation tone, release PCC.

Connect to the WiFi Access Point ESP8266Vario, no password needed.

Now access the url http://192.168.4.1 in a browser. You can use http://vario.local with any OS that has mDNS support. MacOS has built-in support. For Ubuntu, install Avahi. For Windows install Bonjour.

You can now configure an external WiFi Access Point SSID and password. Then you do not have to switch between your home WiFi network and the vario Access Point to be able to configure the vario. After configuration, restart the vario and trigger wifi configuration mode again.

Now if the external Access Point is available, the vario will connect to it as a client, and then start the configuration web server. If your OS has mDNS support, use the url http://vario.local for configuration. Else you will have to watch the serial monitor to find the dynamically assigned IP address.

If the configured external AP is not available (or configured with wrong credentials) the vario will fall back to the stand-alone Access Point and web server. So you can still configure the vario in the field.

Compiled with Bluetooth support :

Compiled without Bluetooth support :

Use Sketch->Export compiled binary to export the compiled firmware binary file to the Arduino sketch folder.

Put the vario into Wifi configuration mode, access the url http://192.168.4.1/update (http://vario.local/update).

Upload the new firmware binary .bin file. Switch the vario off and on again. Select WiFi configuration mode again. If the firmware revision string has been updated in the new firmware binary, you can confirm the updated value in the home page url http://192.168.4.1 (http://vario.local).