I have build a standard opto-isolator (4N37) circuit with input coming in on pin 1 &2. The opto is powered by 5V to Pin5 through a 10K resistor, Pin4 is grounded, Pin6 is grounded through a 400K resistor.
When the input side of the opto is low, Pin4 has a voltage, but when the input goes high, Pin4 doesn't have voltage. It is reversed from what I would expect, the other side of the opto would only have voltage if the input was high. I'm sure this is something basic that I am missing and don't understand. Basically, the opto seems to be working as its picking up the changes in one circuit and switching but not in the manner I expect.