Yes I can see that ADC is definately the way to go. I had a bit of a go with what you suggested and made some headway however could not get totally predictable results. I think the PIC18F4620 is not the best one for me to use to get a good understanding of what is happening. It's datasheet is 130 pages long and it is a bit daunting for a beginner. For every question I research I seem to end up with another long list of questions.

I am going to work through the examples on http://www.rentron.com using the PIC's and examples they have there. Hopefully, that will give me a good understanding of what is going on then I will try again with the PIC18F4620. Will keep you posted.

Cheers Bruce