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.
1 Like