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[Reply #15 on:]

I might get some voltage surges and would not like to fry my arduino, also its a good oportunity to use them for the first time. Can you help me out on how to choose the appropiate one?

You could put in a protection diode for that. Get a 5V Zener diode and connect it in parallel with the lower resistor. If a voltage higher than 5V appears at the resistor junction the diode will short it to ground.

If the resistors have large values (75k, 5k) then there'll hardly be any amps, it should be enough protection.

[Reply #16 on:]

fungus in parallel? like this... see picture. or maybe as the second option i have drawn?

dhenry thank you so much for the explanation. will reread it a couple of times for fully understanding it.

dc42, you are right, that is max. thanks !

[Reply #17 on:]

mm...maybe i understood it wrong. maybe like this?
and the internal pullup resistor from the arduino active.

[Reply #18 on:]

D4 is redundant in this circuit, unless you accidentally short the input of the opto isolator to the output. It might be a useful protection device if you were not using optical isolation. It is also connected the wrong way round.

[Reply #19 on:]

the internal pullup resistor from the arduino active.

That pull-up "resistance" is too high and you may not be able to turn off the phototransistor and generate a logic 0.

Use a resistor and its value will generally be < 10k.

[Reply #20 on:]

D4 is redundant in this circuit, unless you accidentally short the input of the opto isolator to the output. It might be a useful protection device if you were not using optical isolation. It is also connected the wrong way round.

thanks, i thought this is what fungus ment to be able to give aditional protection in case there is a surge in the 80Vdc.
dont understand what he ment then.

[Reply #21 on:]

the internal pullup resistor from the arduino active.

That pull-up "resistance" is too high and you may not be able to turn off the phototransistor and generate a logic 0.

Use a resistor and its value will generally be < 10k.

ok i just say the internal pullup resistor of the arduino is 20Kohms.
I will use a 10k then ! thanks for the information dhenry

[Reply #22 on:]

the internal pullup resistor from the arduino active.

That pull-up "resistance" is too high and you may not be able to turn off the phototransistor and generate a logic 0.

Use a resistor and its value will generally be < 10k.

From the data sheet, the off-state collector current of the 4N35 is 50nA max at 25C. So the internal pullup resistor will be fine unless the OP puts the opto isolator somewhere very hot, or he needs a faster turn-off time (but we're only talking about a few hundreds of microseconds at most).

[Reply #23 on:]

Thanks dc42, i will try both internal and external pullup resistor and decide. I dont need such a short timing.
The circuit so far...

Thanks, any additional information is welcome.
Still trying to figure out where the zener would be at use, maybe its after the R1 to ground.