Show flashing div depending on the result of a query

I have a PHP page that every x seconds consults another PHP the number of rows of a query. Based on that answer, that is, if the value is greater than zero, I would like to show a div , which is an alert, which should be intermittent.

I have half accomplished it, I need to load the page to check if there are more than 0 rows and only show the div with the number of rows if it is greater than 0. Also give some more verbose flicker than I achieved. After the page loads, I would like it to be verified every 60 seconds.

I copy what I achieved, the most important thing:

function actualizar() {
  $('#sensor').fadeIn("slow").load('prueba1.php');
}

setInterval("actualizar()", 5000);

#contiene_sensor {
  position: absolute;
  text-align: center;
  overflow: hidden;
  top: 10px;
  right: 320px;
  width: 80px;
  height: 80px;
  z-index: 29999;
}

#sensor {
  position: relative;
  line-height: 80px;
  display: block;
  top: -82px;
  left: -2px;
  margin: 0 auto;
  max-width: 40px;
  font-family: "Arial Black", bold;
  color: white;
  font-size: 26px;
  text-shadow: -1px 4px 29px rgba(150, 150, 150, 1);
  z-index: 30001;
}

#luxecita {
  z-index: 30000;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>

<div id="contiene_sensor">
  <img id="luxecita" style="width:80px; height:80px;" src="https://placehold.it/80x80/green" />
  <span id="sensor"></span>
</div>

And test1.php has the following code:

<?php

if ($result=mysqli_query($conn_alertas,'SELECT COUNT(*) FROM alertas')) {
    $rowcount = $result->fetch_row();
    echo $rowcount[0];
    mysqli_free_result($result);
}
mysqli_close($conn_alertas);
?>

In case it was not understood: