Auto Logout And Redirect To Login Page

Question

 Hi,

 I need code for auto redirect to Login page when session expires.
 here need to consider the following things.
 1)when page is in idle mode[if session expires auto redirect to login screen and kill the session]
 2) when user performing any action increase the session time.

 Note : when page in idle mode then only auto redirect to login screen when session expires.

 How it can be possible?

 Please help me.