Okay thats a problem since i cant add a pull up to all pnps anymore. (75 in total at 10 registers) Guess i need to switch on the arduino before i switch on the power of the logic... I wonder because i can disable all my pins by pulling the OE to high when everything is working. I Always do this befor i add new data to the registers. When i am done i enable them again. And i can disable all by manually pull the OE up when the arduino is offline...

Having OE low and MR low (pull MR down with resistor, hold it high with Arduino) will force the outputs all to 0. MR sets the contents of the SR to 0, and OE low outputs all that 0 to the outputs - although that requires a toggle of the STCP pin, so forget that ;)

You could have the Arduino control the power to the shift register section (Relay, FETs, whatever is suitable for your currents).

That could work as you say with eh MR. Are you sure about the OE? the Datashet says something else if i get this right: cdn-reichelt.de/documents/datenblatt/A240/74HC595%23STM.pdf And i wonder why i can disable all outputs by pulling it up.

Not a good datasheet - try this one: nxp.com/documents/data_sheet/74HC_HCT595.pdf - look at the truth table - when OE is HIGH: "shift register clear; parallel outputs in high-impedance OFF-state"

But why does it work if the arduino is up and i it is not up i can enable them like this? The MR low and OE low doesnt work either.. It seems that i have to enable/disable the Vcc of the board. 5V and 6A something around this.

Try pulling it into your "off" state with the Arduino, then tickling your bases / gates with your finger - see if you get random switching happening.

Thanks for joining

yw

why can i disable everything with pulling it high if everything is running but cant dissable them like this if the arduino is offline?

shouldn't it work the same?

the OE

Are you sure you're really turning it off, and you're not just by chance seeing the bases get pulled low slightly better when the arduino's connected?

yes i am really sure. Inside of my routine i can disable everythign by setting OE high

i also do this in 8Khz intervalls befor i shift out new data to the registers

Disable the outputs with the Arduino, then measure the voltage WRT ground on the outputs

give me a second

from a output Q0 to gnd

yep

0.00

Now do the same Q0 to Vcc

also 0.00

or 0.01

High impedance.

If it were low you'd see ~5V there

huh yea but why does it disable everything when its running

Like I said - pure chance.

and does not disable when the arduino is offline

LEave the Arduino connected up, but remove the OE connection

You'll see it's probably just the same.

nope everything went up

i got a routin running which shift out to the register

if OE is undefined (as it is without a wire) it just works as "regular"

but if i pull it low several times it never has a undefined state.. its always "off"

i mean high not low

So with OE pulled up with the resistor the voltages on Q0 are what?

i mean if i dont pull it up if i just add 5V to OE

Q0 is like i add 5V from the arduino

okay as expected i guess

i tried pulling it up to 5V. But with 2k i just get 1,6V on the pin if the arduino is still connected to but has no power

Ah right, so without the arduino connected to OE it is in HiZ, but with the Arduino connected to OE, and unpowered, it's no longer HiZ.

You need to prevent back-powering through the OE connection to the Arduino.

It sounds like the Arduino is acting like the lower half of a voltage divider.

Try connecting the OE to the Arduino through a small diode.

(Arduino) -----|>|--- (OE)

i dont got a diode here. Just a regular "light diode"

i think that a problem

Hmmm... you need to get more bits in your experimenting kit then :P

yes i think i need to. So you think if i add a diode with something like 4V break thrue it should work?

with the pull up

Add any small signal diode - the voltage of it doesn't matter that much, but lower = better. A small schottky would be good.

If you have a spare NPN transistor kicking around you could experiment with that.

Connect collector and base together, and you have a diode between B and E.

yea i already thought about the NPN but i just got small pnp and huge N Mosfets here. Working on a 5x5x5 rgb led cube which i control with registers

Not sure which way roound it goes though ;)

k

okay. Guess we figured it out that it's backpowering thrue OE

Sounds like it, yes.

oh well. Add it to the answer that we figured it out and ill accept it and see if i can get it working the days. (Tomorrow ill be in a lab and see if they got some diodes flying around)

Diodes are everywhere ;)

maybe add a small schematiks so i dont forget what we were talking about. Yea i know but i dont got a "kit" i just bought everything myself and forgot those. Just bought stuff for the cube like hell alot of resistors and so on

everything is working.. just the auto off doenst

and my powersupply just has 2,5A but the whole leds would take 5A so if its undefined i could burn it

Yup

Already on it :)

Thank you very much for your time!

going to leave now. Maybe ill let you know if you were right ;)

Cheers :) Good luck.

Oh, you might want to pick up some small NPN transistors too - I've put an alternative on the answer as well using an NPN. I use the BC337 all the time, but the 2n2222 is also very popular. Always good to have some kicking around.

Thanks alot

i guess i got som 2N3094 at the labor.

i am using the 2N3096 PNP so the 3094 NPN would fit.

Good plan.

thanks again!

You're welcome.