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555 with 74HC595 - no microcontroller

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N0AGI

New Member
hi everyone
as a way of intro: I am new to this site and a relatively new electronics hobbyist.

A quick summary of my circuit:
I built a simple circuit with a 74HC595 w/ 8 output LEDs. I am looking to hook up the QH' back to the shift register's input pin. But, have not been able to get it to work. The circuit works just fine just as a basic shift register w/o the "ring" capability.

This below set up works as a basic shift register
The circuit uses :
  1. 555 astable as a clock pulse into the 74HC595/SRCLK/main Clock/p11,
  2. a separate 555/monostable/debounced switch as input into the 74HC595/SER/Data/p14,
  3. and an NPN as an inverter for the 74HC595/RCLK/Latch/p12.

I took the QH' pin (p9) and connected that back to p14/Data and that does not seem to work as as ring/loop back. In my limited datasheet reading/knowledge, it appears that I might need to insert a timing delay before I loop back. What I mean by that is, in looking at the timing diagram, The QH' goes HIGH at the 1/2 time mark of QG/high-pulse window and remains high through QH's 1/2 high-pulse. The QH's high pulse remains high after QH' falls low. My newbie brain tells me that I might need to wait till after QH falls to low before I can connect the QH' back to the input. .

Question:
Can someone please comment / clarify and provide any pointers to me or example of how one should get this to work please.

thanks
 

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crutschow

Well-Known Member
Most Helpful Member
How does it "not work"?

But in order for the loop-back to work you need to initially reset all the registers to zero and then input a single-clock pulse to start the loop.
Otherwise there's no way to determine what bits are being recirculated.

A much easier circuit to do that is the CD4022 4-stage Johnson counter.
It will automatically generate an 8 sequential output with no additional timing.
 

N0AGI

New Member
The MR is pulled low at startup and one high pulse is sent into the SER pin. I understand by pulling MR low will clear all bits to begin with.

I dont have a 4022 - but, can look into that too. I am eager to get the 595 route working and learning from it.

thanks for the input
 
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