.htaccess to 'CLOSE' my website

here is an extract of my .htaccess file which I use to close my website down.

This is controlled througha  PHP script which re-writes the file at the click of a link e.g. on the admin page I can click 'Close' to shut teh website down for maintenence/update etc.

=====
RewriteEngine on
RewriteCond %{REQUEST_FILENAME} !down\.php [NC]
RewriteRule ^(.+) http://something.com/error/down.php [R]
=====

However, this obviously redirects access to any page to the error document.

What I would like to do, is exclude the 'admin directory' from this redirect so that i can still login to admin to 'OPEN' the website again.

If i've been unclear just ask.

Thanks in advance