Using a preexisting pattern for a printer, he reworked it with new hardware and software enhancements to print out letters in braille, calling it the Braigo. The Braigo uses a controller that scans the alphabet. You choose a letter and it prints it out using a thumb tack as a printer head onto on a roll of calculator paper.
The first prototype isn't fast, but it's a concept that works, and works well. Banerjee is working on enhancements that will allow it to print full pages of text.
You can view more information and enhancement updates at the Braigo Facebook page here.