johnsonsec
@brian-johnson · Joined April 2026
IT and security admin · software · 20 years in the field
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Ordered an S type in an alumina tube and a MAX31856. Rule is in, the attribute trick works nicely, set max_c to 1240 tonight and the firing peaked at 1224 with no alarm. A hardware over-temp relay is going in next weekend, fair point. @Aoife mostly drift and the probe’s life, from what I’ve read since.
On the data side: a device profile allows 10 telemetry messages a second by default. At 200 a second you blow through it and a device that keeps doing that gets its session closed.
He’s wrong about the Hermetry half. Hermetry can send commands and shared attribute changes to devices behind a gateway: the gateway gets them on v1/gw/rpc or v1/gw/attributes and has to apply them, so it depends on whether the contractor’s gateway software handles those topics. A command to the gateway device itself works too, so the box in your electrical room can get “start validation sampling” from a dashboard button or a rule and fan it out locally. That’s one command instead of 30. The radio half is Eric’s territory.
That’s a device credential. It logs a device in, it doesn’t read anything for you. The management API wants an API key. Also every 5 seconds is a lot for a buzzer.
Text only shows as the latest value, it isn’t charted. With 0/1 you get occupancy per hour for free with the hourly average.
We did 60 bays for a shopping centre car park last year with plain LoRaWAN and it’s been fine. You don’t need anything special for a pilot. What we built:
Gateway is a Basics Station gateway on a lamp post (the old UDP packet forwarder is not supported, so check yours speaks Basics Station). Sensors added as OTAA devices. If you have 40 to do, pairing mode saves a lot of typing, but it needs a shared AppKey on the profile, which our puck vendor wouldn’t give us, so we added all 60 as OTAA devices by hand. Not fun, budget an afternoon.
The vendor only shipped a JavaScript decoder. Hermetry doesn’t run decoder scripts at all, codecs are JSON tables. Ours was a layout frame on FPort 2 and took 15 minutes:
{
"id": "t.bay-puck",
"title": "In-ground parking puck",
"vendor": "Example maker",
"fports": [2],
"kind": "layout",
"frames": [{ "fport": 2, "min_length": 3, "fields": [
{ "at": 0, "field": "occupied", "fmt": "u8" },
{ "at": 1, "field": "battery_mv", "fmt": "u16be" }
]}],
"tests": [{ "name": "car present", "fport": 2, "hex": "010E74", "expect": { "occupied": 1, "battery_mv": 3700 } }]
}Send occupied as 0/1, then charts can draw it. For the map we made each bay an asset with its coordinates under a “Main Street” site and placed the sensor under it, then a map card on Status under an asset. Free/occupied colour comes from the site status.
Last thing: a Device offline rule on the profile. With a 6 hour heartbeat, use Recent message with interval 21600 and factor 2, otherwise dead pucks hide for days.
Strings over I2C waste bytes and you only get 32 per transfer. Show the master side, I bet that’s where it breaks.
If you script that export, give the API key the Read-only preset and restrict it to your laptop’s network. Seen too many full-admin keys in Python notebooks.
UWB anchors give you decimetres but cost a lot per room and need calibration. For room-level, BLE is plenty.