Kalpak - we're not saying that you are wrong in suggesting an opto - that could be part of the solution. But what was described to us is something that is technically fairly simple but has some fundamental problems. So ,as an engineer, you look to find the mechanism causing the problem. This all sounds like text book stuff - but I am amazed at the number of people that ignore this (myself included on many occasions!). Too many times I see a patch to get around something more fundamental, and many times the patch causes another problem and on it goes. Experience tells us that something should work as expected, but if is isn't, then a closer examination is necessary.