Scotty, thanks for the fast response and the suggestion. I did not appreciate the LDR was sensitive enough to detect different colours. Your suggestion would certainly make the project simpler. For the last hour I have been reading up on LDR's as I have never used one before. I have noted 2 things. Firstly, these LDR seem to be generally used with a RGB LED, although I tend to agree with your thought that a white LED should be sufficient for my project. Secondly, and more importantly, DoI have some concern that they will not be fast enough. They seem to have a rise/fall time of about 30mSec. The ball is traveling at approximately 1mm/1mSec, so the ball is likely to pass the LDR before the LDR has the time to fully react. Of course I can minimise the potential problem by putting the LDR near to the top of the slot where the balls velocity will be at its lowest.
During my read-up of LDRs, I came across what seemed to me to be a similar device, a Light to Voltage Converter, such as a TSL260R-LF. I believe these have a much faster response time of less than 1mSec, which would easily be fast enough for my needs, but I appreciate that they are significantly more expense than the GL5516/5528/5537/5539 type of LDR.
Do you have any experience of the TSL260R-LF or similar product please?
You suggested use of a MCP3008 A/D. I need 9 channels. Should I use 2 off MCP3008, or are there 10 or 12 channel devices available? I could not find any.