This is a very simple and very reliable fully automatic chestcart unloader, when there are no more items in the chest the cart is automatically returned.
How it works
Track detector sends a signal which is inverted by the redstone torch to de-activate the piston
Chest cart sits on the hopper unloading items
When there are no more items in the hopper, comparator de-activates, signal is inverted through the redstone torch and the piston pushes the minecart to return from where it came.
Note:
Track detector placement is important, to close to the hopper and the piston wont retract in time and the cart will return without unloading. To far from the hopper and the piston will retract and activate before the cart can get on top of the hopper. Minecart speed is not critical, as long as it can get up to the hopper
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