smithops
@tsmith · Joined January 2026
Plant manager · automotive · 6 years in the field
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Not at 140 m next to VFDs it isn’t.
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.
We do exactly this for the line lights at our plant, from a little script on the supervisor’s PC. It’s the Commands API with an API key.
Your switch card already sends a command, and boards can only send commands that are in the device profile’s Commands catalogue at the Basic tier. So the method exists. To see its exact name and params, ask the API with the same key you’re about to create: GET /tenant/<tenantId>/api/iot/devices/<deviceId>/rpc-commands lists what a control may send to that device.
Create the key under API Keys: give it the permission to send basic commands and to read commands, put your desktop’s address in the IP allowlist, and pick an expiry. Then:
#!/bin/sh
# ~/bin/office-lights (chmod 700)
KEY=$(cat ~/.config/hermetry/key)
curl -s -X POST "https://<your host>/tenant/<tenantId>/api/iot/devices/<deviceId>/rpc"
-H "Authorization: Bearer $KEY"
-H "Content-Type: application/json"
-d "{"method":"light.set","params":{"on":$1},"timeoutMs":10000}"A two-way command with a timeout of 30 seconds or less waits and answers 200 with the status, so you see SUCCESSFUL or the error in the terminal. Bind office-lights true and office-lights false to two shortcuts and you’re done. An API key can only send methods that are in the catalogue, which is what you want here anyway.
Docs: Commands API.
Feasible. Put the Pi and the router on a small UPS though, otherwise the Pi buys you nothing during load shedding.
Divider on echo, good. How far is the sensor from the water when the tank is full?