Add values of drop down menu( select option) to mysql table

Here is my code

    <?php    
    echo $row['valveName'].'<td>'.$row['valveId'].'<td>';
    echo '<select name="onoff">';
    echo '<option  value="ON">ON';
    echo '</option>';
    echo '<option value="OFF">OFF';
    echo '</option>';
    echo '</select>';
?>
    <tr>
    <td>
    <input type="submit"  name="Submit" align="bottom" border="2" >
    </tr>
<?php
if (isset($_POST['Submit'])) {

    $change=$_POST[onoff];
    echo $change;
}
    /* $selection=$_REQUEST[onoff];
    switch($selection)
    {
    case "1" :
     $change="ON";

    echo $change;
    break;
    case "2" :
    $change="OFF";
    echo $change; 
    }
     */
  $query_auto="UPDATE valve set position='".mysql_real_escape_string($change)."' where valveName='$select1'";
    mysql_query($query_auto) or die(mysql_error());42

but $change has a null value

thanks