Sometimes you only want certain items entering a container. This is when you can use a setup as follows:
The redstone torch at the bottom locks the bottom hopper by default, preventing it from pulling items down from the top hopper. The top hopper should contain 41 items you do want to be pulled down and pushed into the chest, and 4 items that are stackable to 64 but never enter the system. (You can either pick an item you don't ever put in or rename one on an anvil to make it unique.) This makes the comparator output a signal strength of 2, which fades before reaching the dust on the leftmost block one level lower.
When a 42nd item enters the top hopper, the comparator's signal changes to 3, activating the redstone dust on the leftmost block, powering the repeater, turning off the redstone torch and unlocking the bottom hopper to pull an item down, reducing the number of items in the first slot of the top hopper to 41, reducing the comparator's output to 2, unpowering the leftmost redstone dust, turning off the repeater, turning on the torch and locking the bottom hopper again to prevent further item movement.
This way the number of items in the top hopper will never be higher than 45 for long, and only items in the first slot will be pulled down while there are more in there.
Running a hopper line above the hoppers configured to pull in a single item type, will make all items flow past all columns and be processed by each filter. As hoppers only have a small inventory, it's usually preferable to have a larger container as the input, like here on the left.
The chest on the right in this example collects items that weren't picked up by any filters. If this fills up, further items may clog up the input line. So it is advisable to empty this regularly, or lead the end of the input line into a system that destroys items if you're certain you don't need items you put in and don't fit in any chest.
After learning this, I found a plugin to do this in a much smaller way, that also only needs one item for the configuration and no filler items:
As the original isn't particularly difficult, but does take up a lot of space, and the copper hopper is about as expensive as the full build, I figured it wasn't too much of a cheat to use this. You can find more details about this version here: https://www.curseforge.com/minecraft/mc-mods/copper-hopper
