I have tried to create a system that detects if there are less than 14 items in a chest, and give off a redstone signal if there is. If there is no items, there will be a different signal. If there are more than 14 items, another signal.
Now, I tried to do this with comparators, but it only detects LARGE amounts of items - 2 stacks is +1 redstone length.
If you are using some kind of filter (like a hopper filter apparatus) to pull things out of the chest, couldn't you use other items (like sticks or cobblestone) to fill, say, the last slot or two until all you need are 14 items in the first slot to generate the redstone signal from the comparator? But as for creating a 3 signal system (<14; ==14; >14) ... I'm going to have to think about that one some. I'm assuming you want to do this strictly with vanilla redstone resources? Perhaps some more details would help concerning what exactly your trying to accomplish.
I know this is a little old, but it may still be relevant. So I figured out a way to do what you were originally trying to do, but only if we are measuring a hopper-and-chest inventory system and not just a chest. The reason is that a single chest can only give you two signals (essentially a 1 or a 0, so either >=14 and <14 or >14 and <=14). That's easy to do. You just use 110 other items (I used sticks in the images below), so that when 14 of your desired items (cobblestone in the image below) enter the chest, a comparator signal increases from 1 to 2 (the lamp in the images below signal when the number is >=14).
But you were wanting a system with three distinct signals (>14, ==14, <14). That is a good deal more complicated, but it is possible if we use a hopper as an overflow inventory to measure when the chest and the hopper together have more than 14. Below are a couple of images of a system I designed. There are some quirks with the system, but basically it works. The bottom lamp is on when the chest has less than fourteen cobblestone, the middle when it has fourteen cobblestone, and the top when the overflow hopper (the one above the chest) has items in it. The chest will never have more than fourteen cobblestone in it. (I have another system next to it that also works with its own quirks).
I'll write up a more in depth description of the system and post it to my blog in the next week or so (next Monday's post is already taken, but maybe later next week or the following Monday). I'll post a link here when it appears.