LTV-846 & ATMega Port

[Quarlo Klobrigney]

LTV-846 & ATMega Port or what stupid thing am I doing

I have a 12VDC source feeding the diode input to an optocoupler via a 560Ω resistor.
The diode cathodes to ground.
The input to the AVR ports are inputs and their prospective pullups are on.
The emitters of the opto transistors go to ground. The same common ground as the AVR.
What I see is 12VDC on the port inputs!

WHY???

[jwet]

The ground for two sides shouldn't be shared- this is whole idea of an opto isolator.  The LED cathodes on the left should all return to the 12V supply.  The emitters on the right should all return to your micro controller ground.  The only coupling should be the photons that are lighting up the transistors.  The ground of your 12V system has no relation to the ground of your AT Mega.  This is exactly why optocouplers are useful, they can bridge different ground domains.  Their isolations specs (5000V for the LTV-846) is the difference in ground that they can handle.  With the two grounds tied together, you're defeating the purpose of having an opto coupler.