Really, in 99% of cases, you don’t need nor want a trigger. Why???

Any button, or anything inside of the update panel that cases a post-back works like a regular page lifecycle.

So, if you want the UP to update say due to a button click? Then just make sure the button is inside of the UP and you are 100% fine. (no triggers required).

So, say we want a count down clock of some type.

So, in the update panel, drop in your text boxes (to display the time). Add a button to start, and even one to stop the timer.

And MAKE sure you drop the timer inside of the update panel.

Remember, you can think of a Update Panel as a self contained web page inside of the current page.

Keep in mind while a update panel LOOKS like there is not a post-back?

There is!!! In fact even the page’s "load" event fires and triggers each time!!

The correct term here is what we call a "partial" page post-back. So, the full page life cycle occurs here, but ONLY for the things inside of the Update Panel.

So, lets drop in 3 text boxes and a start and stop button inside of a update panel.

Say this:

    <asp:ScriptManager ID="ScriptManager1" runat="server"></asp:ScriptManager>

    <div style="padding: 20px">

        <asp:UpdatePanel ID="UpdatePanel1" runat="server">
            <ContentTemplate>

                <div style="float: left">
                    <asp:Label ID="lblHours" runat="server" Text="Hours" Font-Size="Larger"></asp:Label>
                    <br />
                    <asp:TextBox ID="txtHours" runat="server" Text="0" CssClass="tbox"></asp:TextBox>
                </div>

                <div style="float: left; margin-left: 20px">
                    <asp:Label ID="lblMin" runat="server" Text="Minutes" Font-Size="Larger"></asp:Label>
                    <br />
                    <asp:TextBox ID="txtMin" runat="server" Text="0" CssClass="tbox"></asp:TextBox>
                </div>

                <div style="float: left; margin-left: 15px">
                    <asp:Label ID="lblsec" runat="server" Text="Seconds" Font-Size="Larger"></asp:Label>
                    <br />
                    <asp:TextBox ID="txtSec" runat="server" BorderStyle="Solid" CssClass="tbox"></asp:TextBox>
                </div>

                <div style="clear: both; height: 15px"></div>
                <asp:Button ID="cmdStart" runat="server" Text="Start" CssClass="btn btn-info" />
                <asp:Button ID="cmdStop" runat="server" Text="Stop" CssClass="btn btn-danger" Style="margin-left: 20px" />

                <br />
                <asp:Label ID="lblDone" runat="server"></asp:Label>
                <asp:Timer ID="Timer1" runat="server" Enabled="False"></asp:Timer>

            </ContentTemplate>
        </asp:UpdatePanel>

    </div>

Note how we dropped the timer control inside of the panel. A timer control is actually like a button click – it will "post back" the page based on the interval – it really not much different then dropping a button on the page and clicking it.

Note how I have the timer as disabled to start out.

So, now the user can enter values in to the text boxes, and hit the start button.

So, the start stop code (code behind) is this:

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load

    If Not IsPostBack Then
        txtSec.Text = "5" ' setup a starting value
    End If

End Sub

Protected Sub cmdStart_Click(sender As Object, e As EventArgs) Handles cmdStart.Click

    ' take entered values - convert to time
    Dim MyTime As DateTime
    MyTime = TimeSerial(txtHours.Text, txtMin.Text, txtSec.Text)

    ViewState("MyTime") = MyTime   ' save our time
    Timer1.Interval = 1000  ' upddate every one second
    Timer1.Enabled = True   ' start the timer.
    lblDone.Text = "Started"

    cmdStart.Enabled = False  ' disable our start button

End Sub

Protected Sub Timer1_Tick(sender As Object, e As EventArgs) Handles Timer1.Tick

    Dim myTime As DateTime = ViewState("MyTime")

    ' check if we are to stop
    If myTime <= TimeSerial(0, 0, 0) Then
        Timer1.Enabled = False      ' turn off timer
        ' do whatever for end of timer
        lblDone.Text = "Done!"
        cmdStart.Enabled = True
        Exit Sub
    End If

    Dim OneSecond As TimeSpan = TimeSpan.FromSeconds(1)

    myTime = myTime - OneSecond
    ' update text boxes
    txtHours.Text = myTime.Hour
    txtMin.Text = myTime.Minute
    txtSec.Text = myTime.Second
    ViewState("MyTime") = myTime

End Sub

Protected Sub cmdStop_Click(sender As Object, e As EventArgs) Handles cmdStop.Click
    Timer1.Enabled = False
    cmdStart.Enabled = True
End Sub

Edit: Here is the style I used for the 3 boxes

So, you can drop in this style for the boxes right after the form tag

eg this:

        <style>
            .tbox
               {font:x-large bold;border-style:solid;
                height:34px;width:37px;
                text-align:center;
               }
        </style>

So, the result now looks like this:

Edit3: Example working code based on quesiton

Ok, so the posted markup is this:

    <asp:UpdatePanel ID="updPnl1" runat="server" UpdateMode="Conditional">

        <ContentTemplate>

            <asp:Timer ID="Timer1" runat="server"
                Interval="1000"
                Enabled="true"  >
            </asp:Timer>

            <asp:Label ID="lbl1" runat="server" Text="THIS IS MY LABEL" />

        </ContentTemplate>

    </asp:UpdatePanel>

And code behind is this:

Protected Sub Timer1_Tick(sender As Object, e As EventArgs) Handles Timer1.Tick

    lbl1.Text = "UpdatePanel1 refreshed at: " & DateTime.Now.ToLongTimeString()

End Sub

So, now we see this: