Hi all. I have a Pi 4 that reads soil moisture and a couple of DS18B20s at our community allotment on Saturdays, and the rest of the week it sits at home on the bench reading the greenhouse sensors. It publishes to Hermetry with paho-mqtt.
Problem: it only ever connects to my home WiFi. At the allotment I turn on the hotspot on my phone and the Pi just doesn’t join, so I come home with nothing in Hermetry for that day. I followed a guide and put both networks in /etc/wpa_supplicant/wpa_supplicant.conf with priority=, but it looks like the file is ignored.
$ cat /etc/os-release | head -1
PRETTY_NAME="Debian GNU/Linux 12 (bookworm)"
$ nmcli -t -f NAME,TYPE con show
preconfigured:802-11-wireless
lo:loopbackSecond thing, related: if it’s offline for a bit, can I keep the readings and send them later? Right now the script just drops them when publish fails.
Bookworm uses NetworkManager now, so wpa_supplicant.conf isn’t read anymore. nmcli is the way.
Kevin’s right, and the “preconfigured” connection is the one the imager created from your home WiFi. Add the hotspot as a second NetworkManager connection and give both a priority. I carry a Pi between two factories with exactly this.
sudo nmcli con add type wifi ifname wlan0 con-name allotment
ssid "Aisha-phone" wifi-sec.key-mgmt wpa-psk wifi-sec.psk "********"
sudo nmcli con modify allotment connection.autoconnect yes connection.autoconnect-priority 10
sudo nmcli con modify preconfigured connection.autoconnect-priority 20
nmcli -f NAME,AUTOCONNECT,AUTOCONNECT-PRIORITY con showHigher priority wins when both are in range, so home first. Turn the hotspot on before you power the Pi; if it boots first, give it a minute, NetworkManager rescans.
For buffering: keep the readings in a small SQLite table or even a JSONL file, and when the connection comes back send them with their own time stamps, in batches:
[{"ts": 1762668000000, "values": {"soil.bed3": 31.5, "air.temp": 12.1}},
{"ts": 1762668300000, "values": {"soil.bed3": 31.2, "air.temp": 12.6}}]ts is milliseconds UTC. Delete a row only after the publish is acknowledged (QoS 1, wait for the PUBACK in paho with wait_for_publish()).
One trap with a Pi: there’s no battery-backed clock. If it boots at the allotment and gets no network for a while, the clock is wrong until NTP syncs, and readings stamped with that clock end up at the wrong time or get dropped (more than 10 minutes in the future is refused). Only stamp readings after timedatectl show -p NTPSynchronized says yes, otherwise send them without ts and let Hermetry use the arrival time.
I bought a cheap RTC HAT for this exact reason, DS3231, a couple of quid. Then the clock is right even offline.
And keep the batches reasonable, the default is 10 messages a second per device. A big backlog in one go gets dropped.
Sorted. Added the hotspot with nmcli as tsmith wrote, priorities 20/10. Saturday it joined the phone within about 40 s of boot. Readings went into a JSONL file while the phone had no signal at the far beds, then went up in batches of 50 when it came back. First week the times were off by an hour on the first few readings, the NTP thing exactly, so I’ve ordered a DS3231 like Michael. Until it comes the script checks NTPSynchronized before stamping.
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