Using another board's LED leads as input

I need to read LED state from another board. They are in a 5V circuit. I assume I wire from the + lead of the LED to a digital input port on the Arduino, and have a couple of questions if that is so.

Should I remove the LED and only go to the Arduino's digital port or can I detect that state and still let the LED light up?

Do I need a resistor or another component in line?


It all depends exactly on how it is wired, is it sourcing or sinking current. In general you only get 1.3v across an LED when it is on and that is not enough to show a difference between logic one and logic zero, so yes you need to take the LED out. There is no need for a series resistor.

If it were me I would put a small resistor going to ground behind the led and a wire between the led and the small resistor going to the input. I wouldn't want to ask the input to sink that much current just on principle, I don't know if it would be too much for it to handle.

I wouldn't want to ask the input to sink that much current just on principle

If it can source it, then it can sink it, on this processor. However if you remove the LED you can also remove the resistor and connect the output of one arduino directly to the input of another. Connect the grounds together though.

ah, okay. In what situation would that not work? I notice "on this processor"

In what situation would that not work

On any device that has a source current rating different than the sink current. For example most TTL logic can sink about 20mA but only source about 8mA. In general logic outputs can normally sink more than they can source, this processor is some what of an exception.

However in the context of this question it is likely that they are driving an LED by sourcing current. Inputs don't take much current anyway. So just connecting an input to an output pin will work with hardly any extra loading. If you do that then there is no need to disconnect the LED as the extra load is minimal.

Thanks! I am building a radio direction finding kit that uses LEDs laid out in a circle to indicate the direction, so it is even simpler than taking them out - I just won't put them in. ;D