The opto's are a result of a failed first try. I had planned to just use transistors to turn things on and off, but when I was playing with test versions the results sucked.
Even a 'rail to rail' comparator or op-amp won't quite get all the way down to zero volts when low. The output was enough that the absurdly sensitive computer was turning on and off randomly.
With the opto-isolaters I got consistant switching, due to their lower gain and higher turn-on voltage.
As for the $$$, Digikey has 'em for under a dollar.
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