Quote Originally Posted by Bdlhome View Post
Now I really am going mad or just confused...
My money is on "going mad"

To my knowledge no zener diodes are light sensitive, the LED you are monitoring could react to the light, I don't think that is your problem here though. Perhaps you could put up your code you are using. Do you know how these LED's are being controlled? Something different with this one? What value resistor "fairly large" are you using?

I would simply read from the LED cathode through a 10K resistor to the input pin. Put a 22K resistor pulled down to ground also to that input pin. You may be able to use lower values on the resistors without the pull down resistor turning on the LED, but I would think this would work. Hope this helps some...