This is a digital Attack/Release or Attack/Decay module. It's simple to use with just two pots and two toggles as controls and a single input and output.
This PDF contains schematics, parts list and other information useful for building this module
Boards are available now. Price for 1 board and PIC is $14.
There are unfinished aluminum Eurorack panels available for this project. They are $8 each. For more information on my panels, check this page. If you'd like to drill your own, here's the template that I use, print it out at 300dpi