I've had a bulk feeder in my chicken coop (similar to the one in the Instructable below) for some time now, and I too was wanting to incorporate a visual indicator without having to periodically lift the lid to check.
It is a problem with a setup like this in that you can become complacent after a while (speaking for myself of course) and you finally discover that the container is completely empty when you are greeted at the gate by the starving hordes.
Hunting around for ideas, i stumbled upon this Instructable published around six years ago.
I thought that there has to be a better way though, than installing ropes and pulleys to do the trick. I also don't like having the indicator upside down i.e. 'E' on top and 'F' at the bottom. Call me fussy if you wish.
After giving it some thought, I came up with the idea of having a magnet sitting on top of the food and using a ball bearing rolling down a clear external tube mounted at the side as an indicator of how much food was within the feeder.
Step 1: Materials Used in Construction
Above is my feeding station prior to the upgrade.
1. Block of wood to make a plug to fit into my 100mm PVC food container.
2. Magnet - I used a rare earth NdFeB (Neodymium Iron Boron) N35 which are very strong.
3. Length of clear PVC pipe - I had an old broken aquarium gravel cleaner made from clear PVC that would be ideal for the tube. You probably don't have an old gravel filter, but you can buy clear tube from an acrylic manufacturer relatively cheaply.
4. Ball bearing. My tube was 35mm diameter so I went for a 25mm bearing.
5. String or chain the length of the feeder to connect the plug to the lid. You could use whatever you like here, I used ball chain.
All I had to purchase was a ball bearing $1 (you could probably get a used one for free if you know a mechanic), and the magnet - $15 from my local electronics store.
The rest was from parts I had stowed away in my shed as I tend to be a bit of a hoarder.
Step 2: Making the Plug.
My bulk feeder was made from a length of 100mm diameter PVC pipe which is used for domestic drain pipes. I began by cutting out the wooden plug to house the magnet with a 98mm hole saw. The plug fitted snuggly within the pipe allowing little room for it to drop sideways and get stuck.
Mine was made of two pieces out of 20mm thick timber. A further 30mm hole was made in one of these before gluing them together. This is to form a well to house the chain that would be connected to the lid of the feeder.
Step 3: Fitting the Magnet
My magnet had a diameter of 19mm and was 30mm in length. I, drilled a 19mm hole in the side of the plug and inserted the magnet, making sure it sat in flush with the side of the wooden plug. I didn't have to glue this in as it was a tight fit.
Attach the ball chain to the plug and connect the other end to the lid of your container. My feeder sits external to the coop, so I used silicone to seal the hole in the lid as it is exposed to the elements.
Step 4: Clear PVC Tube to House the Ball Bearing
With that done, the next step was to mount the tube on a treated pine backing board, which I would mount beside my feed container. I painted one side of the clear PVC pipe black to help highlight the ball bearing and painted on some calibration marks.
Step 5: A Lightbulb Moment - Literally
I was at this point when I thought I would take the project to another level and go one step further by adding a flashing light to ensure it would definitely have my full attention when empty.
I already had a 12v power supply in my coop to operate my automatic dusk to dawn chicken coop door - If interested, see my first instructable - https://www.instructables.com/id/Dusk-to-Dawn-Aut...
I had this micro lever switch which I had salvaged from a washing machine sometime back. The idea was to mount this below the pipe where it would be activated by the weight of the ball bearing.
After mounting the switch, I attached two washers to the switch lever as it wasn't quite long enough to be activated by the ball bearing.
I also had an old unused flashing LED bike light lying around in my shed which ran on 4.5v (3 X AA batteries). To incorporate this into my setup, I did some googling and found that I could do this quite easily and cheaply by using a 7805 voltage regulator ($1.85 from nearby electronics store) to reduce the 12v down to 5v which would safely power the light without blowing the LED's.
Step 6: Testing It Out
It was all then just a simple matter of wiring this up to my existing 12v power source.
Here's a quick demo of how it will work using a vacuum cleaner to simulate the dropping level of the feed.
Well at least some of my chickens are happy.