How to detect an external led (on / off)

Other Hardware Sensors
1

Hello,

First of all, Let me say that I don't know too much about electronics. Sorry for my ignorance or vocabulary.

I would like to detect if a led that is attached to an external device is on or off. I managed to solder some cables out of the device. So now I need to figure out how can I detect if the led is on or off with the Arduino.
I noticed that between the Anode and Kathode I have 2 Volts when the led is on (I checked with a multimeter)
My first try was to connect the Kathode to Arduino Gnd and the Anode to A1 and read the A1 analog input. I don't know why but when I do that, then I read 4.7V between the Anode and Kathode no matter if the led is on or off.

The setup is attached.

What is the best (and easy) way to detect if a led is on or off? By the way, I don't have too much hardware at hand (like diodes or transistors).

Is it possible that the GNDs are different and this is creating some problems on what I was trying to implement?

Thanks!

leddetect.png
leddetect.png721×208 9.25 KB

2

If grounds are different and/or dangerous voltages are involved, use an opto coupler.

Connect the LED of the opto coupler in series with the LED of the device.
Connect the opto transistor collector to a digital pin and the emitter to Arduino ground.

In setup, use pinMode(pin, INPUT_PULLUP);
Leo..