Sensor template for last update of security alarm

I am trying to make a sensor to identify when my alarm has gone offline.
It should update every 10 seconds, but if the last update is more than 60 seconds ago, I want to indicate that it is offline.
I have made a template that calculates the time difference between now() and the last update. But the problem is that is only updating every time the “last_update” sensor changes, and when the alarm goes offline, the last_update sensor never changes and my template never updates… mission failed… : (

How do I make the sensor to update every 5 seconds?
Can I use this service somehow?

  - service: homeassistant.update_entity
    entity_id: sensor.example

Here is my code so far,

        friendly_name: "Alarm connected"
        value_template: >
          {% set value = states('sensor.alarm_last_update') %}
          {% set last_updated = as_timestamp(strptime(value, "%Y-%m-%d  %H:%M:%S")) %}
          {% set seconds = now().timestamp()-last_updated %}
          {{ seconds }}

Your template uses the now() function which means the template will be evaluated, at the very least, every minute (on the minute) in addition to whenever thestate value of sensor.alarm_last_update changes.

ah, ok. Then I have to disconnect the alarm to check : ) thanks