The optically isolated output PCB is a very simple construction. There are two reasons why I made a separate board in this case.
This form factor allows for a more modular design. You can have as many outputs as you can find I/O pins on BeagleBone.
It is also cheaper to order small 3.8" x 2.5" mini boards from ExpressPCB than to order a larger custom-size board.
Assembly of this board is very much the same as that of the first board.