What you'll need
1 Toilet fill valve; I used a fluid master $10
1 Toilet supply line $3 (make sure it has 1/2" female pipe thread connector.
1 1/2" male pipe thread by 3/4" female garden hose adapter $3
1 Light socket with switch $3
1 9' 2 prong extension cord $2
All other parts used bucket, nuts, bolts, washers, light bulb, piece of metal and zip ties, I had in the shop.
Take the bucket and make 5 marks 9" up from the bottom of the bucket as evenly spaced as possible. ***After a couple months of testing, I've found that the 9" from the bottom hole location is a bit low and can be subject to overflowing. That said, make the hole location 10 inches or higher from the bottom. This gives more adjustment variant for the valve.*** It's important that the measurement/marks are as level as possible. Once marked, get a 2 1/2" hole saw (it's a drill bit about $8 if you don't have one) loaded in to your drill. Put the pilot bit on the marks you made and drill out the holes one by one being careful to keep the pilot bit on your mark. These will be the drinking holes for the chickies.
Get the toilet fill valve out. With a hack saw, cut off about 1" of the threaded rod on the bottom of the valve. This will allow the valve to sit lower in the bucket. Try to cut the threaded part as straight as possible. Next thread on the supply hose. Next drill a hole for the pipe to garden hose adapter, you'll need to dry fit this so you can see how much slack or lack there of is in the toilet supply line. This will help you locate where the hole needs to be for the fitting. Toilet supply lines are hard to find in any other length than 12". Once the hole is drilled, use some teflon tape on the 1/2" male pipe threads and screw it in to the hole you just drilled in the bucket. Attach the loose end of the supply hose to the pipe fitting.
With the valve in the bucket, stand it up as vertical as possible. Eyeball or measure if you wish for the metal bracket. Bend one end of the metal bracket down and one end up (like a flattened out "S"). Use zip ties to connect one end of the metal bracket to the upper part of the shaft on the fill valve. Be careful that the float (the black cylinder in this case) does not hit the metal bracket or drag on anything. This float turns the water on and off and needs to move freely.
Bolt the other end of the bracket to the bucket. Install optional light socket and wiring.
Once it's all assembled, take it out to the coop. The bucket needs to be placed on level ground, very important. Once placed on a hard level surface, connect the garden hose and turn on the water, slowly. The bucket will begin to fill. With the lid off, adjust the valve (see manual for the valve) float level. Ideally you will get the water up to the very bottom of the holes. You will need to watch the holes to find out which one is lowest as this one will be the first to over flow. This hole is the one you need to adjust the float to. Once adjusted, the valve will come on when the water level decreases (gets consumed by the chickies). In my test, I removed 12 ounces of water then the valve came on. This won't work so well when it gets below freezing as your garden hose will freeze (I'm working on a modification for this)
Here's what it looks like filled up with water.