@TomGeorge
I fully agree. Do turn the relay bords. That's really good practise You're telling.
Railroader:
@TomGeorge
I fully agree. Do turn the relay bords. That's really good practise You're telling.
OK, you guys have convinced me.
Currently printing a new bottomplate (will take 16 hours) and exchanging the diodes.
Hoping to have the new setup (fully according to your suggestions) ready for testing wednesdayevening.
Thumbs up! And have a look if there's any more tips being valuable to incorporate.
beukwood:
OK, you guys have convinced me.
Currently printing a new bottomplate (will take 16 hours) and exchanging the diodes.
Hoping to have the new setup (fully according to your suggestions) ready for testing wednesdayevening.
How are you mounting the three PCBs?
If you use their mounting holes, all you have to do is turn the relay PCBs around!!
Tom... 
I don't want to sound overoptimistic but the first test with just a single 8-relais module looks extremely promising (looks really like it is stable)
Attached a picture of the current setup. I will also install the second 8-relais module and run the arduino for a longer period...
I must admit that installing the 16 diodes was a pain in the ... since the space to solder was terribly tight.
(I also broke 2 out of the 32 connections to the valves since they are apparently really sensitive to bending them up and down..)
Therefore i might try to reduce the 3 improvements down to the necessary one or two, but the fact that your suggestions seem to make a difference is really promising!
Thanks for the suggestions and i keep you updated around the progress.
Can You post a close picture to the troubling connections that want to break? Bending pins etc. is always risky. Never bend it twice like forward and backward. Metall fatigue, known from old aeroplanes, comes quickly.
Railroader:
Can You post a close picture to the troubling connections that want to break? Bending pins etc. is always risky. Never bend it twice like forward and backward. Metall fatigue, known from old aeroplanes, comes quickly.
It are actually the connections of the valve which i bent. I was expecting that they could handle some more flexibility but they do not.
To give you an impression: here 2 photo's how i had to solder the diode kind of on top of the valve. Sometimes there was some epoxy that i had to remove or i had to deal with the plastic cover of the connections... I guess you get an understanding why i sometimes had to bend a connection upward to be able to solder them but sometimes they broke off.. These are actually 2 pictures of easily accessible valves but the ones in the middle at the bottom of the 4x 2 grid are the most challenging..
Thanks!
Looking at the first, upper picture I suggest that You bend the shoe, connecting to the valve.
Start by bending the shoe. If that works crimp the cable to it.
Those contacts are quit rigid and think You could manage even without the isolation, or apply that heat shrinking "sock" plastic, up on the cable before crimping. Than pull the "sock" down and heat it.
It looks like the contacts common in old cars, and other equipment. I assume they are quite hard and not flexible at all.
Regard a shaking motor in a car having the oil pressure sensor connected by such a "nail". It sustains vibrations and must be stiff.
Note that mounting the diode on the solenoid is actually the wrong place to mount it if the concern is to avoid interference with the operation of the logic circuits. The only reason to mount it there is a matter of wiring convenience.
Paul__B:
Note that mounting the diode on the solenoid is actually the wrong place to mount it if the concern is to avoid interference with the operation of the logic circuits. The only reason to mount it there is a matter of wiring convenience.
I strongly disagree. Always kill a disturbing signal as near the source as possible, is my opinion.
Railroader:
I strongly disagree. Always kill a disturbing signal as near the source as possible, is my opinion.
Why do you disagree?
Do you actually understand what the source of the "disturbing signal" is in this situation?
Hint - it is not the inductor! 
Paul__B:
Why do you disagree?Old knowledge learned on the way to a MSc in electronic engineering, confirmed during plenty of years as professional.
Railroader:
Thanks!
Looking at the first, upper picture I suggest that You bend the shoe, connecting to the valve.
Start by bending the shoe. If that works crimp the cable to it.
Those contacts are quit rigid and think You could manage even without the isolation, or apply that heat shrinking "sock" plastic, up on the cable before crimping. Than pull the "sock" down and heat it.It looks like the contacts common in old cars, and other equipment. I assume they are quite hard and not flexible at all.
Regard a shaking motor in a car having the oil pressure sensor connected by such a "nail". It sustains vibrations and must be stiff.
Thanks for the tips Railroader. Well spotted that it are actually the connections from CARS. Since i wanted to have this as a project that would last for multiple years i believed a shoe could prevent a bit of rust and expand the lifetime of my project. (I was also expecting that i would never had to do any kind of activity close to that connection... boy what was i wrong 
The sock is actually part of the connection so you crimp it on there in one go convenient as long as you don't have to put a diode there i guess i have some nice suggestions from your side how to deal with them!
Additional tests for station 2, 3 and 4 are planned for coming weekend!
I started tampering with car wiring 50 years ago.....
For the diodes some extra thinking can be used due to the limited space. It's no absolutely necessary to install the kick back diode that close.
Soldering some short wires to the diode and let this extra wire be crimpted at the same time as the ordinary relay cable could be one way to go. The diode and the solderings needs to be isolated.
That's my intention that things should work "for ever", not only for 5 minutes..... Being reliable.....
Railroader:
For the diodes some extra thinking can be used due to the limited space. It's not absolutely necessary to install the kick back diode that close.
It certainly isn't - it is actually the wrong place anyway.
Paul__B:
It certainly isn't - it is actually the wrong place anyway.
i still disagree.... Why run the discharge current from the electromagnetic field "all around the place"?
Railroader:
i still disagree.... Why run the discharge current from the electromagnetic field "all around the place"?
Because you have failed to discern where that "discharge current" actually is. OK, here goes!
Whether with solenoid or simple inductor, I find it surprising that even educated engineers can resort to "magical thinking" about the situation, with assertions about "current surges" and putting the diode as close as possible to the inductor because the inductor "generates" the surge.
That turns out to be an absurdity. What generates the transient is not the inductor but the switching device, either a mechanical contact or a semiconductor. The inductor - as a response - acts to maintain the instantaneous current flow by generating the "back-EMF". So you provide an alternative path for it to do so through the diode. It is still the case that the current through the inductor and its connecting wires does not change rapidly.
What does change rapidly is the current through the switching element and the power supply which suddenly drops to zero, and the current through the diode which as a consequence suddenly rises from zero to maintain that same current.
The significance of this is that if interference is going to be caused by electromagnetic radiation from a suddenly changing current, that suddenly changing current is located in the loop formed by the power supply (or the local bypass capacitor), the switching element and the diode but not the wiring between the diode and the inductor. The need is thus to minimise the length of that supply - switch - diode loop by placing the diode as close as possible to the switch and power supply (bypass). It is these three that must be close together. Suggesting you need to place the diode close to the inductor (or motor) is actually quite wrong! 
On the other hand, there is a voltage transient caused by the switching which can capacitively radiate interference. This impulse is - again - caused not by the inductor but by the switching element so it actually radiates - possibly counter-intuitively - in the direction from the switching element to the inductor however to all intents and purposes, all points on the wire connecting switch, diode and inductor experience the same transient so this is not affected either way by the location of the diode.
So now let's take a look at that transmission line. 
I won't consider resistance; the simple model of a transmission line is a series inductor and parallel capacitance. The capacitance can perhaps be considered at each end. Well, the transmission line inductance is in series with the primary inductor. If you put the diode at the switching device end, then the inductance acts with the primary inductance and as described, resists sudden changes in the current which is to say, minimises inductive transient radiation.
If you place the diode at the primary inductance end however, you have now created in the transmission line, a second inductor in series with the primary which will add to the voltage transient at the switching device and enhance inductive transient radiation. It may not in itself contain enough energy to damage the unprotected switching device.
Capacitance at either end of the transmission line will indeed serve to slow the voltage transient but will conversely cause a transient when the inductor is switched on. Capacitance toward the inductor end will tend to cause radiation from the transmission line while capacitance at the switching device end will increase more the current surge seen by the switching device.
Note the general principle that it is the transmission line which radiates interference due to switching transients and the current transients occur in that part of the transmission line which is on the switching device side of the diode, so placing that diode near the primary inductor causes all of the transmission line to be such a potential radiator while placing it at the switching device limits this to the loop originally described, formed by the power supply (or the local bypass capacitor), the switching element and the diode.
From a prior discussion.
@Paul__B
Thanks for the impressive arguments. I don't feel I can shoot them down all over. I'll take a second look and see what response I can give You. 15 years of designing heavy electrical fork lift trucks involved some more environmental happenings to consider that are not covered in Your post.
Paul__B:
From a prior discussion.
Thanks!