Basically an appropriate technology course is typically one that is more associated with architectural and engineering aspects of sustainable/permaculture lifestyles. While a PDC is likely to cover things like harvesting rainwater and touch on natural building, an AT course goes into greater detail exactly how these systems are put together.
Tim Barker will be the instructor for our upcoming AT course - to give you an idea, topics he plans to cover are:
While some of that is in a PDC course, his projects go a little beyond that, using rocket mass heaters to create hot water systems, and creating composting toilets that don't just get the job done, but do so in a way that is code compliant and very low odor. Here are a few pictures from some of his projects:
Kinda cool, right? This stuff gets into the engineering behind some of the concepts you'll have to mull over when building a life around permaculture - how you'll cook your food, wash your clothes, where you'll poop - all of that good stuff.
While a PDC gets into the core concepts of the design and philosophy with food systems, an Appropriate Technology course is almost like a lifestyle course in comparison.
Does one have to take a PDC in order to take an Appropriate Technology course?
Nope! Totally separate courses! Of course, one definitely feeds the other, so it's helpful, but if you're already up to speed on PDC stuff, at least in my opinion, the AT course is the logical next step in your education.