Sergegsx:
hi, sorry about that, I copied the wrong image from google images, just wanted to show the basic setup to connect the optocoupler. in case i was missing something. I have removed the picture so it does not confuse other people.the idea is I have a 60 to 80Vdc input, that I need to detect using a digital pin.
is there any optocoupler or any other method where I can just put the 80Vdc and ground to it, and then get a signal which i can feed to the arduino input pin safely?
If it's just 80V DC then get two resistors in the ratio of 16:1 and wire them in series with the small resistor connected to ground. The voltage in between them will be one sixteenth of the input voltage.
You only need opto-isolators if you don't really know what might be connected to the input or if there's the possibility of big voltage surges. eg. they're used in music equipment which has hundreds of cables with interchangeable plugs which need to be connected in a hurry (usually in semi-darkness).