i am wondering if i should install this pull down resistor to stop the input pin from floating or not, my main concern is external interference causing the opto to light and trigger the input.
It is not needed, as the Optoisolator has a current threshold that's high enough to be safe.
BUT, if that wire is long, and it's unshielded, and there is significant electrical interference/noise in the environment (Say the security guard's walkie-talkie etc) THEN you may want to have a filter/bypass capacitor (say 1000pf) right across the Optoisolator input LED. Just in case.
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