5V Input to a Digital Pin causes constant HIGH even when switched off

Dear All
Thank you for taking the time to look over my question, any help would be appreciated.

My problem is, I'm trying to control arduino actions using an external 5V output that switches on and off, this signal goes into pin 0 on my arduino uno R3. When I switch the signal off the pin reads a HIGH at (4.41 V) when the external signal is on the pin reads HIGH at (5.05 V) so there is a difference but the "LOW" is still too high to be recorded as a low. I drew a quick schematic showing what the part of the circuit looks like.

The code used:

int robotVal = 0;
pinMode(robotVal, INPUT_PULLUP);
digitalWrite(robotVal, HIGH);

Once again any help would be greatly appreciated.
Kind Regards

Will

2014-09-10 11_10_35-1 Schematic - C__Users_William_Documents_eagle_SmallArmFDM_untitled.sch - EAGLE .png

Forgot to mention, I also tried defining the pullup as low

There are some 1k pullup resistors on the board for the serial lines (pins 0 and 1) as
that's the resting state for TTL serial signals.

In general leave pins 0 and 1 forUSB serial I/O, choose some other pin.

Thank you for the response, I chose pin 0 as I'm using attach Interrupt to interrupt loops in my code. Is there no way to get a 0V LOW signal from pin 0 then?

I'm an Idiot I just realized the Interrupt pin is 2, Thank you

UPDATE:
I changed pin 0 to pin 2 and this fixed all my problems. Thank you for pointing out the serial pins.

Replaces any previous function that was attached to the interrupt. Most Arduino boards have two external interrupts: numbers 0 (on digital pin 2) and 1 (on digital ...

That's because Interrupt "0" uses pin 2. (which you just found out)

CannyRat:
Forgot to mention, I also tried defining the pullup as low

There's no such thing as a low pull up. Either is activated or its not.

There's no such thing as a low pull up. Either is activated or its not.

Yes that's true about internal pullups but there is such a thing as a "pulldown" resistor (external, not internal).