A hopper will change a stack of items to a single item if it has nowhere to go.

I expected a hopper to put specific items into a chest.

It did put them in the chest but it changed all other item stacks in the hopper to a single item.

1. Set up a clock by running the current from a comparator in its secondary state into its own side using 3 pieces of redstone.
2. Pull the redstone clock signal into a dropper with a repeater.
3. Fill a chest with any item un-stacked. (chest must be in front of the dropper)
4. Put a full stack of that item in the dropper along with other full stacks of other items.
5. Power the comparator and look in the dropper.

Sorry for the bad quality video. Its the only way i could get it to work.