Hi Kevin,
I have found this one:
http://www.octiva.net/projects/ppm/
I have still yet to build it, but the guy who designed it has built and he seems to have it working good.
My parts are on order so i will be able to play in the next month or so when i get round to it. Let me know how you get on if you build it before me.
VR.
__________________
View my Aquarium & Reef Controller build here:
The Ultimate DIY Aquarium Controller of the 21st Century!!!
LED's, PH, Salinity, Clock, LCD, ATO, Auto Water Changer, Variable Speed Cooling Fan Control and lots, lots more......