(Dashed lines are not yet implemented.)
The core of my trailer monitoring system is a Raspberry Pi running a set of python and bash scripts. These scripts are scheduled with systemd.
Breaking the problem up into a set of independent scripts provides several benefits:
- Parts of the system can be turned on and off independently
- Different schedules can be set for each script (e.g. gps.py polls for changes every second, power.py polls for changes every 10 seconds, snapshot.py aggregates data every minute)
- Temporary failures of some parts of the system don't inhibit other parts of the system (e.g. snapshots will be captured even if S3 is unreachable)
- Addition of new components is straightforward
The hardware components and assembly can be found at https://github.com/joshgubler/trailer-build#electrical.
gps.py periodically queries gpsd, which reads data from a USB GPS receiver. It then formats this data and writes it out as data/gps.json.
trailer-gps.service is the systemd service which ensures gps.py is always running.
power.py periodically queries the modbus interface on my Growatt "all in one" charger/inverter. It then formats this data and writes it out as data/power.json.
The company that sold me the Growatt inverter was kind enough to request the modbus protocol spec from the manufacturer.
trailer-power.service is the systemd service which ensures power.py is always running.
Unfortunately, Growatt uses a common generic vendor and product id, which gpsd attempts to register as a GPS device. When this happens, pymodbus errors when attempting to read data from the inverter. To fix this, the following line must be commented out in /lib/udev/rules.d/60-gpsd.rules:
# rule disabled by atru5 as it matches my power inverter
#ATTRS{idVendor}=="10c4", ATTRS{idProduct}=="ea60", SYMLINK+="gps%n", TAG+="systemd", ENV{SYSTEMD_WANTS}="gpsdctl@%k.service"
snapshot.py periodically reads data/gps.json and data/power.json, merges them together, and writes them out as data/{timestamp}.json. These timestamped files allow me to do data analytics over time.
trailer-snapshot.service is the systemd service which ensures snapshot.py is always running.
sync.sh periodically syncs data/ to an S3 bucket. This ensures that I don't lose data, and provides me a way to retrieve last known state should my trailer disappear and go offline.
trailer-snapshot.service is the systemd service which ensures sync.sh is always running.
websocket.py exposes a websockets interface for clients. The script detects timestamp changes to data/gps.json or data/power.json and feeds the updated data to any connected clients.
trailer-websocket.service is the systemd service which ensures websocket.py is always running.
Currently the only websockets client is the iOS app.
The raspberry pi also has a VPN client running, connecting back to my own VPN server. This allows me to connect to the pi from anywhere using a client-to-client VPN connection.