Looping X secs on and Y secs off a certain time span (how?)

Hello, i need a way to control a plug on/off with cycle timing.
Eg i would like to set “Loop 30 secs on and 40 off for 15 minutes the ShellyXX” (trough the web interface or voice control)

Is it possible with an automation / blueprint and how to control it (start or quick-stop) from the web interface?

To answer your question, Yes.

The Blueprint Exchange is set-up to share blueprints that you have created with other folks.
I don’t see a blueprint here, so I suggest you change the topic on top to Configuration with an automation tag. That is where your question will get the most traction.

FWIW, I changed it to Configuration Blueprints because, like you said, it doesn’t belong in Blueprints Exchange.

It’s relatively easy.

Essentially: trigger with whatever you like. I’ve used an input_boolean helper here, so it triggers when that entity turns on. (you could add an action to turn it off after the loop.
Then repeat the action 13 times (13 x 70 seconds = ±15 minutes)
The action consists of turning the light on, waiting 30 seconds, turning it off again and waiting another 40 seconds.

Create a new automation, go to yaml editor and paste the below example in there.
You can then switch back to the visual editor if you like.

Yaml Example:

description: "Turn light on for 30 seconds and off for 40 seconds. repeats 13 times (+-15minutes)"
mode: single
trigger:
  - platform: state
    entity_id:
      - input_boolean.YOUR_BUTTON
    to: "on"
condition: []
action:
  - repeat:
      count: 13
      sequence:
        - service: light.turn_on
          target:
            entity_id: light.YOUR_LIGHT
          data: {}
        - delay:
            hours: 0
            minutes: 0
            seconds: 30
            milliseconds: 0
        - service: light.turn_off
          target:
            entity_id: light.YOUR_LIGHT
          data: {}
        - delay:
            hours: 0
            minutes: 0
            seconds: 40
            milliseconds: 0