Topic: How to do :min_chars for observ_filed just like auto_complete_field?

Hi,

i have a text field and observ_field like this:

<p><%= text_field_tag :searchtext%><%= image_tag 'loader/comment-loader.gif', :id => 'spinner', :style => 'display: none' %></p>
<%= observe_field (:searchtext , :url=>{ :action=>"live_search"},
:frequency=>0.5,
:with=>"key",
:before => %(Element.show('spinner')),
    :success => %(Element.hide('spinner')),
    :update=>"search-results")%>
<div id='search-results'></div>

how can i implement :min_chars options just like the auto_complete_field? i want to triger the observ process only when the user input more than 3 characters.

thank you.