Optocoupler conducts without LED supply

Question

I'm a beginner in electronics. I am trying to use an optocupler (p621) to switch 12v supply to 5v for digital input.

Here is the fritzing diagram

So from the 12v supply I am using 1 1k resistor into the opto P621. So the LED turns on when the button pressed. Then in theory the 5v from arduino connects to pin 2 as a digital input.

Its working fine without much trouble. But even without Optos LED power, the Opto conducts 5v from arduino to the pin.. I've no idea what happens here.

I checked in serial monitor and it says high for that pin randomnly..

So does opto conducts without led voltage or something else wrong in my circuit??

Schematic View

Answer 1

Typically its best to connect the optocoupler in a low side configuration(rather then a high side. The diagram below should help you restructure your circuit:

^{simulate this circuit – Schematic created using CircuitLab}

Hopefully this way, the internal transistor inside optoucoupler fully turns on when 12V is available and arduino would get 0V. When 12V is not available, Arduino would get 5V at the input.(You can invert the input in logic for the sake of simplicity)