Rule on timer with comparison of variable value

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Hi,

Based on an old post here I found the timer device comparison to trigger things. I would like to make a part of it variable but it does not work.

I have the following rule:

    {
      "id": "timer-test",
      "name": "timer test",
      "rule": "when time of TEST > 300 then dim keuken to 30% and stop TEST and reset TEST",
      "active": true,
      "logging": true
    }

I have the following variables (with variable input device to set children_sleep:

    {
      "name": "children_sleep",
      "value": 3
    },
    {
      "name": "timer",
      "expression": "$children_sleep*60"

WHat I want is to replace the “300” value in the rule with a variable $timer (which is set by the variable input device times 60. I then can create a variable timer to switch my music off.

The rule gives an error if I replace 300 with $timer.

Anybody an idea how to fix this? Other suggestions to get a solution for the problem are welcome as well!