My 100 4n33 optocouplers from AliExpress seem to be defective or fake. First, no good pin 1 markings, so I assumed bottom left when the label is upright but they never let the Arduino pin go higher then 0.7 volts. Take it out and the Arduino pin goes HIGH. I double checked my resistors and they seem correct.
Grind or crack one open to see if there is anything inside.
Did you try them, turned 180 degrees?
Yes, 180 deg made no difference. With a multimeter in diode check mode, there seems to be a LED from pin 1 to 2, and two diode drops from pin 6 to 4 as expected according to the datasheet.
I setup a breadboard to test my 4N33 chips. They work fine on the breadboard but in the pcb the unconnected pin 6 (output transistor base) is left floating and somehow picks up stray voltage which partially turns on the 4N33’s output transistor(s). The only source for this stray voltage I can think of is the left behind solder resin. Adding a 5K pullup on the Arduino’s input helps (10K didn’t work yet) but better yet is to add a large (>100K) pull down resistor on the 4N33’s pin 6. Am I the only one with this issue? Wondering if the PCB designers want/should add this pull down R.