input text onchange event

How do I make a text input fire an event when it is automatticlly updated…

function first_sender()
{
document.getElementById(‘second_sender’) = “done”;
}
function alert_user()
{
alert(“ok”);
}

…

<img src=”none.jpg” onMouseDown=”first_sender()”>
<input type=”text” id=”second_sender” onChange=”alert_user”>

Thank you. please note this is a simplified example of my problem.

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