I am running into some problems with my shift register and I have some questions. I am currently running an UNO to control my common anode 7 segment display with the SN54HC595 shift register. I am powering the leds with a different power supply so I am using the SN54HC595 to sink the current. To start off I have a couple questions with this shift register.
I have 9V applied to each led and each led has a 560 Ohm resister. Each led is connected to a pin on the shift register. To toggle the led on I obviously have to pull the pin low to ground. My question is, since I have to toggle the pin high to turn it off does that mean when I pull the pin high my +9V will have a direct connection to my +5V that is powering the shift register from the UNO? Because when I unplug the uno and maintain power to my leds through my 9V supply my UNO stays on due to what I am guessing could be the power connection of the pins pulled high through the shift register? If thats the case how would I fix it?
Also with these shift registers my hope is to "set and forget" so I free up my processor for other processes (it will be a big project and the leds will not be always changing). That should work correct due to them having latching capabilities?
Due to the amount of leds in this project I need multiple shift registers. However the leds are broken up into different sections so I decided not to daisychain every shift register in series as it would cause too much of a delay to light up the last bit. Basically delay of one shift register X 10 on the last bit is something I want to avoid. However I need to use the least amount of pins as possible. My idea is to have an dedicated CLOCK and SERIAL pin for each shift register but a shared LATCH Pin between them all. I figure I can pull the latch pin high and low as much as I want on the shift registers I am not trying to update so long as I don't touch their CLOCK and SERIAL pin correct? So if I want to write to Register 1, I address the CLOCK and SERIAL pin for that register and toggle the LATCH pin. Register 2, even though connected to the shared LATCH pin, should still maintain its current states since I did change anything to its own dedicated CLOCK and SERIAL pin right? This would allow me to cut down from 30 required pins to 21 if that works while maintaining individual control of each shift register.
I have been reading that an LED driver would be best for constant current sourcing however if I am only powering 2ma leds on each pin (aside from the common anode 7 segment display 20ma leds) would it not be best to stick to the shift register as its far cheaper and only get LED drivers for the 7 segment display? I am sinking every led by the way.
Also when I am controlling the 7 segment display I get some really faint ghosting on the off leds. I am not multiplexing it and I have a delay of 5 seconds before it updates again so it really shouldn't cause ghosting due to the delay and not constant switching right?
Sorry for the long post and thanks in advance!