4n25/35 optocoupler detect led on/off

It might work if you use the Opto coupler's transistor to pull the Arduino's input pin to ground. If the opto is off, the internal pull-up in the Arduino pulls the input pin high. If the opto is on, it sinks current, and pulls the input pin low.

You should use a separate current limiting resistor in case the LED in the opto has a higher on voltage than the one it's in series with.

Mike.

im looking to hook up the 4n25 to an hm-10 led:

If you connect your opto directly across the blue LED (Vf ≈3.3V) it will hog all the current and the blue LED will never light. If that is ok then do it that way. If not then connect the opto in series with a resistor between PIO1 and Gnd. Check data sheets for current available on PIO1 and needed by opto.

Mike.

if you current mirro your led ? perhaps by some ratio to opto input ... would be quite fast (i assume)
your source by https://www.google.ee/

◄click to follow
to get - the image applies to https://www.vishay.com/docs/81864/4n25x000.pdf -- ****
then it takes to check the https://www.farnell.com/datasheets/2047068.pdf
to see that the company uses to mark the product from the same family so thet the text is oriented from least / highest pin no. toward the middle ones ...
so it takes one to two test to tell which is the case with 4N25
----------
note **** the datasheet shows characteristic graphs starting from 100µA iR-Led (Transmitter) current (IFW at legends) which has proven to be the case and in practice for xx817xx -s
----------
with uno

You could hitch up the opto like this and still have the blue LED light :