I made one of these but I took a different approach. I based mine off the filter design so when there are at least 9 items in a hopper it triggers a timer and dumps exactly 8 items into the furnace, which is loaded with coal at all times. It then sends a signal to check to see if there is more ore to smelt.
I hooked this up to another automatic filter system that will automatically add coal and the proper ore and then move these to a storage room so all that needs to be done is dump all items into a single chests and it will sort, and smelt efficiently.
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Playing Minecraft since [Friday, March 19, 2010, 9:20:21 PM] (First indev world save)
My only issue with this is that it can't use mixed fuel streams. I've been trying to design one that, basically, checks whether it as seen a smelted item leave the furnace in the past 10.5 seconds. If not, it gives the furnace one more fuel item. The downside is that you have to keep the system packed, in the input, so you can really only use it with one smeltable item (mine is cobble), but the result is that it only can use coal or blaze rods.
Still trying to get it to work, though. If you can come up with a 1 or 2 wide tileable version of this, I'm sold.