Redirect /admin to https and all other URL to http

This is probably an Apache ABC but I do not see the light. I want to my Django admin pages served in https, but only the admin pages. This implies:

• Redirect http://my_site/admin/* to https://my_site/admin/*.
• Redirect all traffic that is https but not part of https://my_site/admin back to http.

After googling I created the following apache rewrite rules

RewriteCond %{HTTPS} off 
RewriteRule ^/?(admin.*) https://%{SERVER_NAME}/$1 [R, L] 
RewriteCond %{HTTPS} on 
RewriteRule !^/?(admin.*) http://%{SERVER_NAME}/$1 [R, L]

Result: Going to http://my_site/admin redirects to https://my_site/admin (hurray) and the browser gives message Error code: ERR_TOO_MANY_REDIRECTS, this webpage has a redirect loop.

Does anybody have a hint where I am wrong? Below the full httpd.conf (made anonymous)

Thanks in advance, Arjan.

ServerRoot "/home/my_account/webapps/my_app/apache2"

LoadModule dir_module        modules/mod_dir.so
LoadModule env_module        modules/mod_env.so
LoadModule log_config_module modules/mod_log_config.so
LoadModule mime_module       modules/mod_mime.so
LoadModule rewrite_module    modules/mod_rewrite.so
LoadModule setenvif_module   modules/mod_setenvif.so
LoadModule wsgi_module       modules/mod_wsgi.so

RewriteEngine On
RewriteLog "/home/my_account/logs/user/rewrite.log"

RewriteCond %{HTTPS} off
RewriteRule ^/?(admin.*) https://%{SERVER_NAME}/$1 [R, L]
RewriteCond %{HTTPS} on
RewriteRule !/?(admin.*) http://%{SERVER_NAME}/$1 [R, L]

LogFormat "%{X-Forwarded-For}i %l %u %t \"%r\" %>s %b \"%{Referer}i\" \"%{User-Agent}i\"" combined
CustomLog /home/my_account/logs/user/access_my_account.log
ErrorLog /home/my_account/logs/user/error_my_account.log
KeepAlive Off
Listen 24047
MaxSpareThreads 3
MinSpareThreads 1
ServerLimit 1
SetEnvIf X-Forwarded-SSL on HTTPS=1
ThreadsPerChild 5

WSGIDaemonProcess my_app processes=2 threads=12 python-path=/home/my_account/webapps/my_app:/home/my_account/webapps/my_app/my_app_folder:/home/my_account/webapps/my_app/lib/python2.7:/home/my_account/webapps/my_app/lib/python2.7/site-packages
WSGIProcessGroup my_app
WSGIRestrictEmbedded On
WSGILazyInitialization On
WSGIScriptAlias / /home/my_account/webapps/my_app/my_app_folder/my_app_folder/wsgi.py