In this circuit i use hcpl 3180 i use 2 in series in my circuit i know that the minimum current for hcpl 3180 to turn on is 10 ma so how this circuit work(if=10ma)

 

Not sure where you got your information about LEDs, but it is wrong. An LED will light with 0.0001 amps, 0.01 (10ma) is the rated current.

 

According to figure 15 of the datasheet the minimum forward current for turning on the output is 3mA.
10mA is the recommended forward current.

 

According to figure 15 of the datasheet the minimum forward current for turning on the output is 3mA.
10mA is the recommended forward current.

what is the function of R2 IT JUST LIMIT THE VOLTAGE OR WHAT
THANKS

 

what is the function of R2 IT JUST LIMIT THE VOLTAGE OR WHAT
THANKS

I don't think it's really necessary. Where does this circuit come from?

 

It looks like R2 limits the diode reverse breakdown voltage.