Comparator needs an blockupdate

When I want to use the comparator to build an key card system then the comparator needs an block update to know there's something in an hopper.
So when I put an item into an hopper then the comparator doesn't go on it's of ,but when I place or destroy a block next to the comparator then it'll react but when the hopper is empty It'll won't go off ,if I destroy or place a block next to it