Limit excerpt length by characters (text with shortcode)

I’m using the following excerpt filter but it limits the post length by words. This filer show shortcode as [ ]. How can I put a limit on characters (text with shortcode)?

<br />
function get_excerpt($count){<br />
  $permalink = get_permalink($post->ID);<br />
  $excerpt = get_the_content();<br />
  $excerpt = strip_tags($excerpt);<br />
  $excerpt = substr($excerpt, 0, $count);<br />
  $excerpt = $excerpt.'... <a href="'.$permalink.'">more</a>';<br />
  return $excerpt;<br />
}<br />