Hi Eric and Gerty
Thanks for your help and sorry for the late reply (for some reason, I don't get reply notifications any more).
Anyway, with regards to the broad spectrum photo diodes, you posted, Eric, won't daylight be a problem, considering that it detecs from 400 to 1100nm?
Gerty: Brilliant link, thanks
It does pose a problem, because I was hoping to make the photo detector small and cheap and, most importantly, able to run on a small battery of sorts, but I'll see what I can work out.