I am building an I/O board for my Arduino controller. The I/O board is flexible to let you use the Arduino I/O as either an input or output via a 4N25 opto coupler. My external I/O are all 24 vdc. The load for an output could vary from 20-350ma. My schematic is posted below. To use the Arduino I/O as an input, the output jumpers are removed and the input jumper is installed. Just the opposite case to use the Arduino I/O as an ouput. Can you look this over and make suggestions or possible corrections. Right now this works on my proto board, but I have a few concerns. My first concern is the base resistor sizing on Q1. I would like approx. 10ma input current to the 4N25 opto. I am not sure I have the sizing right, even though it works. This Base resistor is giving me slightly less than 5 vdc on the base of Q1. My input voltage to pin 1 of the 4N25 opto is 3.8vdc. This I don't understand. Why am I dropping so much of my 24vdc across Q1? Is it not fully on? Any help thoughts or comments would be greatly appreciated.
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