Got to thinking about clocks using droppers and came up with this one. Not like amy I've ever seen before. With the 9-tick repeater clocks driving it it gives a max time of about 40 minutes which can be adjusted to anything up to that max in 18-tick steps.
Although it might not seem obvious, the max number of items you can cycle through it is 10 stacks (640) not 9 stacks. This is becaust the 2 hoppers have a combined capacity of 10 stacksand can be totally filled by the dropper before it switches out. Circuit can probably be improved but I like it.
Now that I'm sitting here thinkimg about this, I see that I can add extra items 5 more stacks at a time by just putting more hoppers in the chain on both sides. Would increase the footprint 6 blocks for each insertion but should have no limit on size except space available. For instance 4 hoppers on each side would allow 20 full stacks of items to circulate.