Hello, i have followed some tutorials here but i still need some help. I want to hook up this optocoupler to a 5V DC source that can give a maximum of 450mA. I don't know if i used the correct values from the data sheet and if my algorithm for calculating the resistor is correct. In the schematics i represented only the diode part of the optocoupler for simplicity. I used the VF=1.25V from and IF=20mA under TEST CONDITION from ELECTRIC-OPTICAL CHARACTERISTICS table as diode characteristics. I first calculated the remaining voltage drop on the resistor and then the value of the needed resistance. The resulted resistance R1=187.5ohms is between this two available values that i could use: 180ohms and 200 ohms. Should i use one of them, and which one if so, or should i group resistor to get exactly 187.5ohms? One more question. Do i need to lower to the current that comes from the source to 20mA or the LED draws only what it needs? I didn't find anywhere any explanation for this kind of situation. PS: Sorry, i forgot the datasheet.