Openhab 4 mqtt device channel no values are reported

Setup, Configuration and Use Items & Sitemaps
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1

Hi all

I try to read values out of a mqtt device (generic mqtt thing) but only get NULL.

I installed the mqtt plugin to octoprint and configured it to send values to my mqtt server - so far so good. When I connect ro ma mqtt server with MQTT Explorer all vallues are shown as they should (eg. {“actual”: 26.16, “target”: 0.0, “_timestamp”: 1749393991})

If a create a thing in openhab 4.3.5 I can read the connected state as it should. so I think the integration seems to be ok. But if I try to read out any other values like the bed temperature, I get NULL only.

I use as topic “octoPrint/temperature/bed” but the I have to use “Incoming Value Transformations” as follow: “JSONPATH:$.actual”.

anyone any idea what I’m doing wrong?

Thanks
René

2

If you can see the correct bed temperature in MainUI and MQTT Explorer, I guess the items definition is wrong.

Here is my config for two of the Octoprint items to compare:

Thing:

UID: mqtt:topic:mosquitto:octoprint
label: MQTT Octoprint
thingTypeUID: mqtt:topic
configuration: {}
bridgeUID: mqtt:broker:mosquitto
channels:
...
  - id: temperature-bed-actual
    channelTypeUID: mqtt:number
    label: temperature-bed-actual
    description: ""
    configuration:
      stateTopic: octoPrint/temperature/bed
      transformationPattern:
        - JSONPATH:$.actual
  - id: temperature-bed-target
    channelTypeUID: mqtt:number
    label: temperature-bed-target
    description: ""
    configuration:
      stateTopic: octoPrint/temperature/bed
      transformationPattern:
        - JSONPATH:$.target
  ...

Items:

Number OctoPi_temp_bed_actual "Aktuelle Temperatur Bett [%.1f °C]" <temperature> { channel="mqtt:topic:mosquitto:octoprint:temperature-bed-actual" } 
Number OctoPi_temp_bed_target "Gewünschte Temperatur Bett [%.1f °C]" <temperature>  { channel="mqtt:topic:mosquitto:octoprint:temperature-bed-target" }

Make sure the Thing id matches the Channel definition.

3

Here my configuration:

UID: mqtt:topic:78c1df2347:Printertest
label: Prusa Printer TEST
thingTypeUID: mqtt:topic
configuration: {}
bridgeUID: mqtt:broker:78c1df2347
location: TEST
channels:
  - id: Connect
    channelTypeUID: mqtt:switch
    label: Connect
    description: ""
    configuration:
      stateTopic: octoPrint/mqtt
  - id: Timestamp
    channelTypeUID: mqtt:number
    label: Timestamp
    description: ""
    configuration:
      stateTopic: octoPrint/temperature/bed
      transformationPattern:
        - JSONPATH:$._timestamp
  - id: HotendTemperature
    channelTypeUID: mqtt:number
    label: Hotend Temperature
    description: ""
    configuration:
      stateTopic: octoPrint/temperature/tool0
      transformationPattern:
        - JSONPATH:$.actual.
      unit: °C
  - id: BedTemperatur
    channelTypeUID: mqtt:number
    label: Bed Temperatur
    description: ""
    configuration:
      stateTopic: octoPrint/temperature/bed
      transformationPattern:
        - JSONPATH:$.actual

It looks quite same, specially the configuration of the bed temperature. I can’t see any significant difference.

Where do I find the code of the item? If I loo to the code in “Edit item” it looks quite different:

label: Bed Temperatur
type: Number:Temperature
category: ""
groupNames:
  - Prusa_Printer_TEST
tags:
  - Point
4

You are using MainUI to create items, I am using *.items files to create them manually.

I don’t know anything about MainUI as I am using text based config for openHAB, sorry.

But if I take a look at the values coming from Octoprint, they don’t appear to have a unit:

2025-06-08 18:53:30.257 [INFO ] [openhab.event.ItemStateChangedEvent ] - Item 'OctoPi_temp_bed_actual' changed from 34.61 to 21.84

So remove the unit and use a plain Number itemtype:

This worked for me:

label: Bed Temperatur test
type: Number
icon: ""
groupNames: []
tags: []
5

Hi,
Do you have install “JSONPATH transformation” ?

1 Like
6

@Tschetan

You’re my hero…

I never read about this addOn while reading or looking to tutorials.
Yes this was the root cause - thanks a lot.

@sihui
It even works with the unit definition. Thanks as well for your hints

René

1 Like