First, I am a programmer, not an electronics guy. But I do try to make some circuits and usually manage OK with digital electronics. But this time I have a microcontroller that switches some signals using a LTV847 4-channel optocoupler and also a LTV817 1-channel optocoupler. I was under the impression, probably from another circuit that I once used that had a 1K resistor on the ground side of the LED that triggers the transistor in the chip. Truth is, I think I borrowed the circuit from someone else's design. Anyway, this time my signal is 3.3V and maybe the other time it was 5V and that might be why I needed a resistor that time. Anyway, I seem to be able to switch the signals OK on the LTV847 but not on the LTV817. I shorted across the resistor and then it worked. Do I need any of those resistors? I would rather not have them in the circuit if I don't need them. Well, especially if they do more harm than good.