Noisy latch signal 74HC595 Daisy Chained structure

Hi everyone,

I have connected 15 74HC595 shift register to control 120 optocouplers through current limiting resistors. Optocouplers works good and allow me to drain enough current.

My problem is noise in latch pin. every 74HC595 IC has its own pcb. I have worked with CD4094 and working with them for about 2 years. I have made an update to my design and used 74HC595 shift registers due to higher current output. Now i will describe what I have, every PCB has a power input 12 V. This power input has 2 caps 470 uF and 100 nF. 12 V power rail feeds into 12 V motors and they use 0.8 A at full power. I have a voltage regulator for each 74HC595. 7805 voltage regulator is connected to 12 V with recommended 33 uF cap and 0.1 uF cap at output side.

The problem is I have some noise in the circuit. If i touch latch and gnd pin of the last 74HC595 and squize it, it gets better. So, im assuming my problem is filtering the latch pin from some noise. Obviously I shouldnt have power noise due to 3 caps 33 uF, 0.1 uF and 470 uF.

I have searched the solution on the web and arduino forum. There are so many suggestions and some of them involves dangering the arduino itself. So I decided to open this topic and get help about it. The datasheet and the ShiftOut tutorial of the arduino suggests that using 1 uF cap on latch pin is enough to filter the noise. But where do I put that single cap ? To the last IC or the beggining of the latch line ? And a frequent suggestion is to use 27 pF at the end of latch line. Another suggestion is to use 74hc245 after 12th shift register.

So, I have so many suggestions and dont know which one is good and reliable.

Moderator added spacing for readability

the ShiftOut tutorial of the arduino suggests that using 1 uF cap on latch pin

That tutorialis wrong, do not do this. What data sheet suggests this?

You need to post a schematic your word description is insufficient.

You have motors involved, do you get the problem if you replace the motors with LEDs?

Are all your 0.1uF capacitors the ceramic type? They need to be.

How about posting a photograph as well as a schematic.