Thanks Admiral. I am pleased that I persisted and got there, and found a definite problem, so I have confidence that it is a real solution. Much more satisfactory than trying something that finally works but not being sure why.
The limitation on my choice of processor comes down to my ability to program it. It is a very accessible system, intended for students and beginners. There are bigger chips in the system, which offer more i/o but apparently more timers. Certainly there is an application note for a PID implementation on the base PIC chips, but I can barely understand how it works, let alone how to adapt it to the Picaxe system, and I don’t have the ability or equipment to program it on a raw chip. Basic language with a three pin plug in circuit is about my limit in the programming area.
I think that the timer availability might have been part of the reason it was recommended by another forum member to use a different processor very early in the project. I saw the possibility in the simple processor, and was interested to see if I could make it work, and while more i/o would have been useful, it was not necessary in the end.
However, I also learned something about that little photo interrupter component and got plenty of soldering practice, along with a new iron after my old one died early in the project. So it’s all good.
Thanks for your encouragement.
MJM460