what is the voltage at the emitter of optocoupler 4N25?
4N25 and quite a few other opto devices have what is known as "current transfer ratio". For the 4N25 this is specified as being greater than 20%.

This means if you stuff 20mA into the diode, you should expect to see at least 4mA through the opto-transistor and your emitter resistor is 1k so you should see something like 4V or possibly a little greater.

If you are putting 10mA into it don't expect much above 2V on the emitter.



This is the table from the data sheet - note that there testing conditions are with collector-emitter voltage at 10V.

If you are not registering any voltage at the emitter, the diode could be broken - try measuring its volt drop after the resistor in series with it. Alternatively you may have connected it up incorrectly. Also check your 1k emitter resistor isn't  something like 1 ohm and check your collector is at 5v.