How to detect if LED is switched on in external circuit using Arduino NANO?

Other Hardware LEDs and Multiplexing
1

I have an external circuit which contains an LED. The circuit is powered by a 5V power supply. The LED uses 2V and 0.4mA.

I want to detect when this LED is switched on using the Arduino. How would I go about doing this? Would I connect the negative terminal of the LED to ground and positive to a digitial/analog pin? My knowledge of electronics is not so good so any help appreciated.

2

Similar discussion going on over here. (Maybe you guys are in the same class :wink: )

3

Would a circuit such as the one below work?

1336×980 155 KB

4

please explain more!
Is there a common point between the external circuit and Arduino? For example, GND?
Which pin LED is connected to the switch. Cathode or anode?
If the ground of both circuits can be connected, you can connect a controller pin of led with a 10k resistor to an analog pins and can check values in two modes ,on and off .

If you can not connect two circuits, you can use opto-couple.