Stratux Zero

Portable ADS-B/Flarm/OGN traffic receiver, AHRS, GNSS, pressure altitude and CO-alarm based on the Stratux project and a Raspberry Pi Zero 2.

The Flarm/OGN can be reconfigured for 978 MHz UAT for non-European use.

This project is based on the great work of:

• Stratux (cyoung/stratux)
• Stratux - European edition (b3nn0/stratux)

The Stratux software for this project is mostly untouched. The main addition is a more user friendly build system with more configuration and customization options. It also has the firewall enabled and does more cleanup and tweaking to enable faster boot time and lower power consumption.

Read more about the Stratux project at stratux.me and at dross:aviation.

Note that this does not replace any primary systems and should only be used as a complement for cross-reference and data collection for post-flight debrief. None of the software or hardware has been tested or certified by any independent agent and should not be relied upon.

THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.

Features

• Dual USB 2.0 receptacles for NooElec NESDR Nano 2+ or similar, Microchip USB2512
• Pressure altimeter, Bosch BMP280
• 9-axis motion tracking with 3x gyroscopes, 3x accelerometers and 3x compasses, TDK ICM-20948
• GNSS (GPS, Galileo, GLONASS, BeiDou) with 2.5 m CEP accuracy, support for GAGAN, WAAS, EGNOS and MSAS augmentation, does not support RAIM, u-blox CAM-M8Q
• GNSS backup power for faster initial lock
• Carbon monoxide detector with 85 dB audible alarm, SGX MICS-4514
• Battery socket that take standard 18650 battery without built-in protection
• 4.2 V overvoltage, 2.5 V undervoltage and over-current protection, make sure to select a battery that is compatible, TI BQ2972
• USB C power with built-in 1 A charger (can be set up to 3.15 A with a resistor change), with support for USB BC1.2, USB Type-C Rev. 1.2 CC, Apple, Samsung and legacy USB charge adapters, Maxim MAX77751
• Can be powered from 4.5 to 13.7 V DC with a connector adapter with a detection resistor
• High and low battery temperature monitoring with charging inhibitor
• Fuel gauge that monitors the battery state of charge and aging, TI BQ27441
• Hard/soft power switch that first signals the Raspberry to shutdown cleanly and if that takes to long, power will be cut
• Optimized for a Raspberry Pi Zero 2
• Reduced Wifi transmit power to limit the range at which the open access point is visible
• Watchdog that resets the Raspberry Pi if it become unresponsive
• System boots in about 11 seconds
• Five LED indicators
  • Green power indicator that flashes every 5 seconds if low battery and 2 times per second if very low
  • Green Stratux status indicator that flashes when when waiting for startup or loss of GNSS lock
  • Red carbon monoxide alert that flashes every 5 seconds if detected and 2 times per second if CO is high
  • Green external power indicator
  • Orange charge indicator that flashes while charging and turned on when fully charged

The CO-alarm, watchdog, charging temperature monitoring, battery and power management is handled by an Silicon Labs EFM8BB10 8051 MCU.

TODO

• Testing of first revision PCB and software
• Add fuel gague check in MCU code
• Add default AHRS device orientation settings in Packer build files
• Light pipe CAM files
• Enclosure CAM files

Contents of this repository

• CAD and CAM files for the custom PCBs
• CAD files for the enclosure
• Source code for the EFM8 firmware
• Source code for the monitoring and shutdown service for the Raspberry Pi
• Hashicorp Packer build files and instructions on creating the Raspberry Pi image file

Design decisions

Because of the global component shortage, many of the components are selected out of availability. This leads to a more expensive design with several difficult component footprints. Still, several of the components are impossible to acquire separatly and may need to be desoldered from various break-out and development kits.

The integrated 1090 MHz antenna for ADS-B data is a simple dipole design. This however, requires a balun with impedence matching that is located on a separate PCB.

Power management

There is a DC/DC regulator that take power from the battery or an external USB-C and output the main 5 V supply. This 5 V powers the Raspberry Pi through the GPIO header and powers to the two USB A connectors.

From the 5 V, a 3.3 V and 1.8 V is created to power various circuitry.

The 5 V regulator can be enabled from three sources.

1. The physical power switch will always keep the 5 V active.
2. When external power is provided on the USB-C. However, the EFM8 will keep the Raspberry Pi from starting using the RUN-pin if the physical power switch is in off.
3. The EFM8 can keep the 5 V active to allow the Raspberry Pi to power down cleanly.

An additional 1.8 V is created from the battery to provide a backup power to the u-blox CAM-M8Q. This enable a faster startup and satellite lock.

Multiple test points can be found on the backside of the PCB.

Ideas for improvements

• Add a USB power delivery switch to the external ports so that they can be switched off when the unit it charging while Raspberry Pi is not started. For example, using a TI TPS2001D.
• Add a third power switch position that would start the unit whenever there is external power. Good for when wanting to start and stop the unit with an external power switch. This can be done by implementing a voltage divider that can be sampled by the MCU.
• Add pin header, MOSFET and diode for a 5V fan that can be controlled from GPIO4.
• Switch to a u-blox with RAIM or a dead reckoning GNSS like NEO-M8U.
• Add battery specifications on the silkscreen visible in the battery holder.
• Move C3 to batter location between IN and PGND for IC2 (MIC2876) to reduce noise from the inductor.
• Add ESD protection for the USB line as the pogo pins make them very exposed.
• Add pull-up resistors to SHDN, PIRDY and PISHDN as the EFM8 only has global internal pull-up.
• Thermally isolate the battery temperature sensor so that it isn't cooled by the board. For example by removing the internal planes and moving the ground via.
• Add more test points, for example on the charge disable.

Building the Stratux Zero image

These instructions are written for a AMD64 Ubuntu Linux environment and can be run in Windows 10 WSL 2.

Prerequisites

Install all required components for compiling and building the Raspberry Pi Zero 2 disk image.

Pulling the optional Drewsif/PiShrink submodule allows the finished image to be shrunk, making the flashing faster.

Install required packages:

sudo apt-get install git unzip qemu-user-static e2fsprogs dosfstools libarchive-tools jq

There is an issue when running QEMU in WSL on Windows that may require you to run sudo update-binfmts --enable after a reboot. This could potentially be added to a script in /etc/profile.d/.

Install Packer:

curl -fsSL https://apt.releases.hashicorp.com/gpg | sudo apt-key add -
sudo apt-add-repository "deb [arch=amd64] https://apt.releases.hashicorp.com $(lsb_release -cs) main"
sudo apt-get update && sudo apt-get install packer

Install Go if missing:

wget -c https://go.dev/dl/go1.17.6.linux-amd64.tar.gz -O - | sudo tar -xz -C /usr/local
export PATH=$PATH:/usr/local/go/bin
echo 'export PATH=$PATH:/usr/local/go/bin' | sudo tee /etc/profile.d/go_path.sh

Build and install packer-builder-arm plugin:

git clone https://github.com/mkaczanowski/packer-builder-arm
cd packer-builder-arm
go mod download
go build
sudo mv packer-builder-arm $(dirname $(which packer))
cd ../
rm -Rf packer-builder-arm

Building the image

The build is done using Hashicorp Packer and a plugin for emulating the ARM architecture using QEMU.

sudo packer build src/stratux-zero-image

The build will take around 20 to 45 minutes and will result in a file named "stratux-zero-raspios-arm64.img". This file can be flashed to an SD card using Etcher or similar software.

Customizing

The image can be customized by passing variables to the packer build command.

Build with SSH access enabled:

sudo packer build -var enable_ssh=true src/stratux-zero-image

If you plan on enabling SSH, it is a good idea to change the raspios_username and raspios_password variables.

Build with developer mode and HDMI output:

sudo packer build -var enable_developer_mode=true -var enable_hdmi=true src/stratux-zero-image

Build with SDRs setup for the US:

sudo packer build -var enable_uat=true -var enable_ogn=false src/stratux-zero-image

The -var can be used multiple times and variables can also be passed as JSON or HCL configuration files or as environment variables.

For a full list of variables and their default values, see variables.pkr.hcl.

Building the EFM8 firmware

The onboard 8051 microcontroller requires Silicon Labs Simplicity Studio for compiling and flashing the firmware. The code is written for Keil Cx51 compiler and uses the Keil RTX51 Tiny real time operating system. A free Keil license can be registered through Simplicity Studio.

Silicon Labs debug cable adapter

To program the board you will need a Silicon Labs USB Debug Adapter and a TC2030-IDC 6-pin Tag-Connect cable. Four pins need to be wired between the debug adapter and the Tag-Connect cable.

Description	Debug Adapter IDC Pin #	TC IDC Pin #	TC TAG Pin #
Not Connected	1
GND (Ground)	2
GND (Ground)	3
C2D	4	3	3
C2CK pin share	5
C2D pin share	6	5	5
C2CK	7	2	2
Not Connected	8
GND (Ground)	9	4	4
USB Power	10