Using A0 as digital input wiht internal pullup

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15 is the voltage being read, must be some offset (will be in the range 0-1023 for 0-5Volts)

A0 would connect (for example) to a switch then to 0 Volts.
But:
If you are using A0 as a digital input, you should have the code look for a 0/LOW (for 0Volts) and 1/HIGH for (5Volts) just like any other digital input.
if (digitalRead(SELECTOR) == 1) // or 0
do stuff
Or
if (digitalRead(SELECTOR) == HIGH) // or LOW
do stuff

BTW, Rather than #define, I would use:
const int SELECTOR = A0;

Vaclav:
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@Vaclav:

How to use this forum

See point 14.

Once you have solved your problem, do not delete the thread! The forum is supposed to be a knowledge-base. Deleting a thread once you are happy does not help future people with similar problems.

You've been on the forum for a while now, you should know the basics.

Have the courtesy to not ask a question, get an answer, and then delete the question.

Vaclav:
Deleted SOLVED

And to Nick's comments I would add "Quit blaming your tools for your own lack of knowledge."