This bubbled up today. He explores some choices he made in building his DIY kit in a series of blog posts on tDCS.
Just to see how easily it could be done, I built a couple of tDCS units for about $30 each using common parts. The meters were purchased from EBay for about $7 each and all the remaining components came from a local Radio Shack, including the case, voltage regulator, resistors, etc. The tDCS units feature a potentiometer to make it possible to adjust current for treatment specifics or pad variations.
(Two tDCS units built in about 3 hours for well less than $100)