Create template sensor of old value from other sensor

Configuration
1

What would be the best way to create template sensor of old value from another sensor.

I have sensor A which has a numeric* value. Whenever this value changes to another numeric value, I want to store the old/from nuumeric value in sensor B.

*ofc I know, that every value is a string, but wanted to point out, that I’m not interested in changes from sensor A to or from unknown/unavailable/whatever, but only need the last numeric number from this source sensor.

Starting point template sensor, so it get restored after restart for the case that A is not available and because there are no changes at this point of time.

But then? I thought about storing every change in the attribute and then iterate over so finde the last numeric state, etc. etc. But I assume, that is far easier and I only can’t see it.

Would appriciate any hint.

2

For a simple “just store last numeric value”:

template:
  - trigger:
      - platform: state
        entity_id: sensor.a
    sensor:
      - name: B
        state: >
          {% if trigger.from_state.state|float('x') is number %}
            {{ trigger.from_state.state }}
          {% else %}
            {{ this.state }}
          {% endif %}
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3

Thanks a lot!

First I thought that this would not work with some nun-numeric values in between. But then I realized, where my knot was.

4

O.k. though again and now see my knot again and where my problem was before in my tries as well.

Let’s assum A has this values in a time row:
1, unavailable, 2, unavailable, 2, unavailable

Then B chain is
unknown, 1, 2

But it should still have 1, because that was the last different numeric value.

5

Tested this: I think it does what you want, where B is a “latching” version of A, retaining numeric values across unavailability; and C is the prior version of B:

template:
  - trigger:
      - platform: state
        entity_id: sensor.a
        to: null
    sensor:
      - name: B
        state: >
          {% if trigger.to_state.state|float('x') is number %}
            {{ trigger.to_state.state }}
          {% else %}
            {{ this.state }}
          {% endif %}

  - trigger:
      - platform: state
        entity_id: sensor.b
        to: null
    sensor:
      - name: C
        state: >
          {% if trigger.from_state.state|float('x') is number %}
            {{ trigger.from_state.state }}
          {% else %}
            {{ this.state }}
          {% endif %}

The to: null prevents it triggering off changed last_changed attributes as described here.

6

Thanks again. Will test it. Thought about a helper as well and was only wondering, if it is possible with only one template sensor.

Know this null. But is it really needed for such “internal” attributes as well? Thought it is only needed if I want to suppress triggering of “real” attributes. If yes, every sensor would trigger on every last_updated as well I was not aware that this is happening.

7

Happy to be proved wrong, but I don’t think so. From your example:

Let’s assume A has this values in a time row:
1, unavailable, 2, unavailable, 2, unavailable

Now assume a 3 comes in next. Where is the prior 2 stored?

Try it without and see what happens. In my testing, it was necessary, but I was using all sort of faked-up sensors with an input_boolean as an unavailability switch.

8

Thought about an attribute, but I’m fine with two sensors of course as well.:+1:

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