Barrel items library with sequencial items withdrawal, help

[Finnen78]

Hello.

Im building an Items library with item pipes and ive done quite well with these color coded item-pipes, but im having a problem with my cobble library, as every miner knows we get tons of cobble.

Ive found a nice way to diminish this stack and turning it into power but i dont know how to build the redpower signal system so that - one redstone engine shuts off when the barrel/chest is empty and continuing with the next one in line. I got some 30 barrels full of cobble. i could simply put a lever and a redstone wire on the ground and run all the engines at once, but it feeds to much cobble.

i have watched a number of youtube videos on my problem but ive found it very hard implementing their solutions into my Barrel library.

how can i make it feed sequecially from each barrel, untill its empty / or move to the next when one cobble has been removed from the barrel?