Fez Panda K8000 induction problem


I have build a domotica system with a FEZ Panda and some velleman K8000 board cards.
The software i use is written by Bart Vanderhaegen.
This system runs perfect on 2 locations.
But on the 3 location it gives me large problems.
By an ukknow reason it swiths output on without a push on a button.
The communication between de FEZ Panda and the 3 K800 cards runs via I²C bus.
The lenght of this wires is max 50 cm.
To make a input on the K800 card you have to but 12V dc on a optocoupler and then the soft decide wich output to swith on.
I have connected al the GND together. The power supply is 12V DC and with a scopp i dont see any problems.
The only diffrent between the 3 installation is the housing of the whole system.
2 housings are made in Steel and the third is made from Plastic.
Can Anyone give me some tips to solve that problem.
I know its not easy but maybe 1 tip can help me further.

Many thanks Filip

Do you have another metal case to try? Seems pretty clear that this is a significant difference, and it “seems” like your problem is caused by electrical noise interfering with the device, so by doing this you will test that theory.

Filip, as Brett said, it sounds like environmental noise.

As a test, you could wrap the device in aluminum foil and ground it.