Ok I just re-read your post and I think I misunderstood.
If you are trying to adjust the brightness of your internal joystick leds, may need a different approach.
The circuit I recommended is completely off or completely on, based on optoisolator output.
Adjusting the brightness of LEDs are not as simple as one may think, however you can try to put a pot (adjustable resistor) in between the optoisolator and the joystick, then adjust the pot to vary the brightness of the joystick leds. (This is not efficient, because at some point the led will drop off fast to dark.)
The best way to vary the brightness of an led is adjust the current (caution! very easy to fry the led), or pulse the current off and on at different rates. I don't know if this is possible with your joysticks or not.
Yeah, I want to switch between two different levels of resistance. One when the opto-isolator is off, another when it is on. The simplest way I thought of to accomplish this was adding another resistor in parallel with the first one when the opto-isolator is on. (I'm using pots so they're adjustable, but for the sake of keeping it simple, we'll just say I'm using resistors).
The joystick PCB already has a pot to adjust the sensitivity. I've removed that pot and replaced it with some pins and a cable that will go to this circuit.
At the higher resistance, the IR LEDs are dimmer and the joystick less sensitive so it doesn't register in the corners. (4-way mode)
At the lower resistance, the IR LEDs are brighter and work normally for 8-way games.\
This is with a round gate on the joystick and it doesn't feel as odd for 4-way games as I thought it might.
I messed around with it today with the joystick hooked up instead of just measuring resistance. After a fair amount of fiddling, I got it working the way I want and moved onto building the actual boards, but I'm using a 100ohm resistor.
(I was mistaken when I mentioned a 220ohm resistor yesterday, it was a 120).
It didn't behave exactly as expected, so a 200ohm resistor might work, but I'd have to remove them from my proto-board to test.
I have a half dozen more opt-isolators. Hmmm...
EDIT: ugh, ended up tearing the proto-boards halfway apart before I realized that the pads on the edges aren't linked to the other side like the holes.
This after I tested to make sure that the individual holes weren't linked in any way. ....and one of the optoisolators is blown.
It's a shame, as it was working exactly how I wanted on the breadboard for around a half hour of fiddling with it. Not sure how long out of that the opto-isolator was powered. Back to the breadboard I suppose.
Is there something other than an optoisolator that would be better to use here?