Connecting Shift Registers with Different Parts

[ydobberstein]

Hello Responder/Reader,

This is my first post. I have looked everywhere on the internet, but needless to say, I have not found my answer. It has helped me somewhat with understanding what a shift register is, but I am no expert. My electronics experience is from a couple of projects, a bit of reading from Forrest M. Mims III's book Getting Started in Electronics, and a bunch of random reading/watching on the internet(EEVblog videos and others). I have been wondering if I could chain or connect multiple different registers to get to the amount of LEDs I need(specifically 20 groups). I have not built anything yet; I am just thinking through what to do in my mind, not that there is much to think about being that my experience with shift registers is null.

I have been working on an LED strip stair project using shift registers and an Arduino Nano. The original project was by Andrew Ong http://speedysignals.wordpress.com/2013/08/01/how-to-assemble-stair-lights/  I modified it.  He had 16 light groups, perfect for the amount of outputs in the registers. I am not so lucky. I have drawn up(actually drawn, pen and paper) a possible way to do his project with 20 LEDs. Sadly, I do not have a scanner. If you could imagine 3 (8 bit 74HC595 Serial In Parallel out) registers daisy chained together you would see basically everything I modified from his version. There are empty outputs on the last register. Would this delay or mess up anything? Anyway, I just don't like it. So could I possibly take a 16 output + a 4 output = 20 output?

Thank You

P. S. if you have any ideas on how to make this better i.e. trash the nano or make it less expensive in anyway, please let me know. Thanks again.

[jlmoon]

ydobber,

You should be able to take the last output from your shift register chain and tie it back to the MR (master reset) after some logic shifting and start the count chain over again. 
What I mean is say you have 3 '595's tied together in serial fashion just take Q5 or whatever Q# on your last device and loop the entire process by resetting all the parts by tying the MR to that last line you're using.. the Q5 ..6 or whatever.  Be sure you're paying attention to the logic sense of MR i.e. Master Reset occurs on a Low level or 0volt state. 

Hope I'm explaining this right. 

[ydobberstein]

jlmoon,
So are you saying to put it into a kind of loop onto itself? Wouldn't that turn the LEDs off then back on? on the schematic on Ong's website it show's a connection from pin 8 to the next IC's pin 14. That's what I was thinking of. Maybe you're method would work. I don't know. I was going for a sort of counting. 1 led, 2 led, 3 led, so on... keeping them on until I reset the whole thing. Also is Master Reset the same thing as SRCLR? Sorry I'm kind of confused. 

[ydobberstein]

Never mind. I see what you mean. Thank you.

[jlmoon]

Been 30 years since I have played with any shift registers.. back in the day we used 74LS74 D flip flops to do the same thing.  when you reached the end of the count / shift chain you reset all and start over.  The outputs / leds should sequence from lowest Q output to the highest then the not(MR) (don't know how to put a bar over that MR pin on here) will clear all and start over again.   If you extend the reset out a pin or two from your last output you will get a delay at end of sequence by the number of clocks it takes to get to that clear function pin.

As far as your cascading function you should have the first IC's Q7S (pin 9) tied to the next inline IC pin DS (pin 14).  You can also blank the entire output chain by taking Pin 13 high on all SR IC's. 

Shift registers are fun indeed!