These are the three versions of MegaMod PCB for Sega Genesis. Created with Eagle PCB.
v1 - uses PIC16F630 (https://vmod.wordpress.com/megamod-v1-0/)
v2 - uses Atmega328P with internal oscillator (https://vmod.wordpress.com/megamod-v2-0/)
v3 - uses Atmega328P with internal oscillator + THS video amp (https://vmod.wordpress.com/megamod-v3-0/)
PIC code used in v1: https://github.com/sebknzl/saturnmod
The Arduino code for v2 and v3 was based on this project: https://github.com/SukkoPera/MegaDrivePlusPlus
Due to some bugs related to the Atmega328P internal oscillator the Arduino code was never finished.
The audio part of the mod is based on the circuits posted in this forum by Ace (MegaAmp): http://www.sega-16.com/forum/showthread.php?26568-Introducing-the-Mega-Amp-The-universal-Genesis-audio-circuit