HowDoI: Convert MQTT received value to a 2 decimal place float?

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1

How do I convert an MQTT state topic from a floating point with 6 decimal places (i.e. 5.145245) to a float with 2 decimal places (i.e. 5.15) ?

The MQTT topic is actually a string but HA (cleverly) converts it to a float. I also have the voltage device class in customize.yaml

Values in the front end are correctly displayed but to 6 decimal places.

This is the MQTT sensor:-

hvac_trvs_energenie_8008:
  sensor:
    - platform: mqtt
      state_topic: "/energenie/eTRV/Report/Voltage/8008"
      name: "Lounge TRV Voltage"
      unit_of_measurement: V
      force_update: true

I believe this creates the sensor.lounge_trv_voltage entity? Well I certainly use that in my lovelace cards and the value is displayed.

2

There may be an easier way, but I would look at a value_template https://www.home-assistant.io/integrations/sensor.mqtt/#value_template

3

Thanks - but that linked article might as well be in Russian . It’s not far off as it is.

My temperature sensor is mapped to an mqtt topic, as above, how on earth do I start to figure out what a value template needs to be. Is there any [better] documentation to help?

4

There is a lot of docs on templating. Seek and you shall learn.

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5

DO something like this:

  - name: "Temperature WZ 2"
    platform: mqtt
    state_topic: office/sensor3
    unit_of_measurement: °C
    value_template: "{{ value_json.temperature | round(1) - 1 }}"

round(2) will give you the right float number back, and you can do simple calculations too

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6

Hi, I found the Template tab on the Developer Tools, I tried this:

{{ states('sensor.lounge_trv_voltage') | float  | round (2) }}

which yields 2.94 in my case. Perfect! float added to force integer to float in the event sensor value is an integer like 1,2,3 …

By the way … round(1) - 1 gave the incorrect value but I get where you are coming from.

7

All states are text, so you are right, the float is needed to convert from text to number type.

8

yes, seek and though shall learn mate !

9

I was too quick. I tried the value_template in the template editor. All seemed well. When I put the code together , viz:-

 - platform: mqtt
      state_topic: "/energenie/eTRV/Report/Voltage/8008"
      name: "Lounge TRV Voltage"
      device_class: voltage
      force_update: true
      value_template: "{{ states('sensor.lounge_trv_voltage') | float  | round (2) }}"

…I always get 0.0

I suspect that it is because the value template is referring to itself? I am at my limits of understanding - for now.

Any ideas anyone?

EDIT: sussed

Just use:

value_template: "{{ value | float  | round (2) }}"

value is predefined and yields value for the current in-scope sensor being defined.

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