I input a 5V AC signal into the input port of PC817 (measured by a 3.5 multimeter). Why is the output port also 3.5V? I saw that the maximum input is 6V and the maximum output is 35V. My output port is connected to 14V. Why is the output only 3.5V instead of 14V
How much current are you flowing through the LED on your input?
With AC input, what are you using to measure the output? DC or AC?