This is one (out of 15 identical) slice of the supersmelter-like XP farm I'm currently finishing, attached to a massive kelp farm. (the rails are drawn wrong direction; the schematics don't support rails "into/out of the screen")
The currently missing bit is how to automatically lock all the hoppers to collect XP. My current mode of operation is to manually place these redstone torches to lock the hoppers to be able to retrieve an item smelted, collect XP, then knock them out and stash in a chest for next use.
The walkways must remain free as they are (redstone dust or other 'intangible' extras are okay though), as sometimes junk will fall into one feed system or another and needs to be removed and they provide just enough access to the lower part of the furnaces to be able to grab some XP. The area below is completely unoccupied though, and a free game.
How can I toggle a whole line of these bottom hoppers in all the slices with a single lever – power them from below?
Best Answer
Since you mentioned the area below is free game, you can use another row of redstone torches directly below your current torch. This would make your current torch turn off as per redstone interactions.
This wouldn't be good, right? So you put this secondary redstone torch over a cobblestone, and, to the side of this cobblestone, you place a repeater, pointing to the cobble. By feeding this repeater a redstone signal, this would turn the secondary redstone torch off, and turn the current torch on. Then all you had to do is link them all with redstone and a lever.
When the lever is ON, it would turn the secondary torches off, and the current torches on. When the lever is OFF, it would turn the secondary torches on, and the current torches off.
Just remember that you need to consider the max range of redstone signals for this to work.
This is a little schematic I could do online to kinda show how it would look. Consider Gold as hoppers, tnt as repeaters and mushrooms as redstone