Ok. so I have gotten to the point where I'm trying to do my overall design with LEDs. I am happy with the LEDs I'm using right now with 3.3KΩ resistors - they are bright enough like that and are using less than 1mA like that. My project will in total now have 22 LEDs. But I have ran into a case where I want some of them running in series - but I think in overall design effect - they are also running in parallel - which from my testing so far and from what I've read is a bad idea (parallel). So I need help understanding what is doable and what isn't.
Here's a sketch that I think will help:
I have seen similar designes to the left circuit for powering LEDs from another PSU so the power draw is not comming from the Pi to run the LEDs. In my particular case - I assed the second LEDs in series on the 3 GPIOs. I have a scenario that really needs to have 2 LEDs in series as they will be at opposite ends of a cable and are inidcators of an active link and of activity on the line (in fact they are LEDs built into RJ45 sockets I'm putting on a main board and daughter board). So, I have done testing with series LEDs like this. My concern is - with a single 3.3KΩ resistor/LED pair - I am getting the current down to less than 1mA on the 3.3V GPIOs - which leaves plenty of mA on the GPIO to run multiple LEDs - BUT - when I put 2 LEDs in series - even with no resistor - they are very dim. Too much Vd on the LEDs? So, then I think - use parallel - feed them the same voltage going in, but then the issue with them pulling power more than one another and that doesn't work. Am I missing something here on how to do this when I have plenty of mA od current left on a GPIO?
Also, based on the sketch - if you take out all the components on the left - GPIO, optocoupler, etc... and simplyfy the net circuit after the GPIOs are turned on - do you not end up with the circuit on the right? And, if so, that is a parallel circuit. What's going on here? Why is it that in one case parallel is bad - but then these circuits are effecivly still a parallel circuit? Is it that the LED pairs have their own resistor connected to the +5V so this balances it - or this this overall still wrong?
This is what I would like to do: