Glad to update this topic because it allows me to give you a feedback.
After having tested several solutions, I came back to the idea of the simple mechanical sprinkler programmer, type "tap nose".
Not having the necessary skills in electronics, I dropped the perilous crafts on electronic programmers, certainly more sophisticated but more expensive too.
On the other hand, the mechanical models are very simple to tinker:
It takes a programmer model with ball valve rather thansolenoid valve with membrane, ie a breakthrough ball that turns 1 / 4 of turn to pass the water.
Attention because I saw that for the same model of the same brand, from one year to another is a diaphragm valve or ball. To know, just look to see the ball that turns or actuate it: if it's a click, it's a membrane, if it's a sound engine, it's a ball.
There are even models with 2 valves, so 2 opening possibilities.
- Prog. watering 2018.jpg (22.66 KIO) Viewed 4829 times
Just gently cut the case on the back with a small hacksaw to get access to the ball, then fix a pulley or rod on the ball (glued and / or screwed) to actuate the opening of a strike, hook or other mechanical system: the ball rotates with a sufficient torque for that.
- Prog valve ball.jpg (40.32 KIO) Viewed 4829 times
In the photos we can see that I even cut the tube on the length of the box and put on a rod that allows me to fix the programmer on a box: I slide the rod in a tube and locks it with a screw.
Advantage:
- economic because the cost is about 20 €
- it's plastic so simple to tinker
- The case is waterproof and small in size
- easy to program: 1 button for the duration, 1 button for the frequency
- There are 2 or 3 batteries of 1,5 volts, type AA or AAA depending on the model, the battery life is about 1 year, but we can add an extra battery pack to increase battery life. A small light, however, indicates the battery level
Disadvantages:
- The programming is "relative", that is to say that we do not program a specific time but a frequency opening from the time the timer is activated : 1h, 2h, ... 8h, 12h, 24, 48h, 72h. For example, if I want to trigger an opening at 7 in the morning, I have to activate the prog the day before at 19h by setting the frequency to 12h. (There are other possibilities).
That said, the new models have a small button Time limit which makes it possible to refine the start time (adds 1 h by pressing the button).
- The shape of the boxes of these prog is often rounded or oval, so a little more complicated to set in a set.
To go back to the horse feeder, I made a big wooden box with the lid that opens with traction springs at the programmed time: Everything worked well ... except that the horses feeling the hay in the subwoofer at night were hitting on it and moving it, then they broke it!
So, I opted for a small box hung on a tree in their park, out of reach of horses, and which opens with the same principle. For now, everything is working!
In addition, what is amusing is that the horses understood the mechanism and put themselves under the delivery time of their hay ... and get hay on their head 
I keep the idea of the box placed on the ground because it is easier to handle: it must be filled in the evening and schedule the opening for the next day or the morning for the evening, but I must find an indestructible box that horses can not move.
Otherwise, this mechanical ball system that rotates at a frequency and duration allows all kinds of applications:
- Small solar follower (described on econology: here )
- Chicken door opening / closing
- Opening of a small feeder for rabbits, chickens, ...
etc
). I still keep this idea of automatic feeder that I will manufacture with a large plastic crate type 
... and then I go back to bed, serene, the duty accomplished! 
) 