Edge trigger via loss of Vs on 555 help

Thanks to Hero for getting me this far!

I want to use the attached for a timer circuit, so far got most of the componants and hurdles as well as the rest of the (exterior to the timer) circuit sorted.

However....

I need to trigger (I think this is an edge trigger?) via loss of Vs (12v). Normally, the 12v will supplied. I want the timer to produce it's output once only when the 12v is cut. Note cut, it isn't switched to 0v, it's simply a loss of a Vs voltage to the trigger. Currently I think this circuit is set up to trigger via a 0v feed when I need it to trigger by loss of 12v (output on break).

Can anyone suggest a modification or direct me to a good web page on designing edge triggering?

Thanks, Greg.

This will work.

P.S.

In furure, please restrict the discussion of a circuit to just one thread; multiple threads can get confusing.