resistor needed for optoisolator?

hello : )

i have a 5v logic signal and wish to trigger an opto-isolator with it . i have tried an on-line calculator for working out a resistor, but need expert help to be sure.

Opto-isolator is a: dual HCPL2731 DIP8

ratings from the data sheet

Input Forward Voltage 1.4v(typical) - 1.7v(max)

i think it says 12ma input max? ( so i've gone for 8ma)


the data sheet is hereHCPL2731 Datasheet pdf - Dual Channel Low Input Current, High Gain Optocouplers - Agilent (Hewlett-Packard)

i worked it out to be 470 Ohm resistor if the desired current be 8ma.

but i could be totally wrong about the 8ma.

thanks for any help. : )

Most "5V" logic signals supply only 3.5V at 0.5mA or less.

You need to know how much actual voltage at how much actual current is provided from your digital signal.

A 470 ohm resistor in series with a 1.5V LED limits the current to (5V - 1.5V)/470= 7.4mA Which is not very bright.

Your math is ok and 8 ma is probably ok if your logic signal can supply that much. You don't say how much current you need from the output but with 8 ma drive you can get at least 20 ma of drive from the opto.

thanks uncle scrooge

The logic signal is from an arduino which can output 40ma max per output pin , i think it is very close to 5v , i shall check the actual voltage tomorrow when i am set up again.

i not 50 percent sure about the actual 'ma' for the opto-isolator now, i can't make out the schematic very well. The Internet shop i'm buying from is saying 20ma max on their web page , but the schematic says otherwise i think .5 min and 12 max so i thought about 8ma would be o.k.

Looks like your pin will source 4.5V at 20mA at 25degC.

thanks everyone

i think I shall purchase 360 ohm resistors when i order the opto's.

really appreciate your help.
have a nice week : )