Isolated output PCB
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.
Isolated output PCB
Assembly of this board is very much the same as that of the first board.
Install the resistors first.
Install the LEDs, making sure that the polarity is correct.
Install the transistors, again checking for correct installation.
Install the ICs preferably on sockets to make repairs easier.
Finally, install the connectors.
Opto-isolated output PCB schematic
All four circuits are identical. A logical 1 on the input applies 3.3V to the base of the NPN transistor. This turns on the transistor and pulls the collector of the transistor low. This turns on the status LED [D1 – D4] and the LED portion of the 4N35...
