# What resistor with Optpcoupler?

Hi I am quite new to the Adruino world and like many others on here looking for some advice/guidance in understanding data sheets. I have read many posts on here and go over the Ohms law numerous times so beginning to get a little more understanding.

I am trying out building a sound activated flash trigger, seen a few posts on the forum about this so have done as much research as I can but am basically looking for guidance on what resistor to use with my optocoupler.

I have managed to get hold of a SFH618A-2 (http://www.vishay.com/docs/83673/sfh618.pdf) now I will be hooking this up to one of the Arduino digital output pins but obviously need a resistor so I don't damage it. Now where I get confused is on calculating the resistor, the Ohms law principles seem simple if you have all the numbers, but from the datasheet I cant really get the simple figure I require? Now I know I have 5v coming from the Arduino board so there is figure number one, but then cant see a forward voltage although I can see a forward current.

Basically I am after someone walking me through this and how they would calculate the resistor value based on this datasheet. Sorry if this is a really simple newb question but like I say I am new to this and don't want to just guess and blow my components.

Yes you need a resistor
Look at the Electrical characteristics , line INPUT :
Test condition : If = 5mA
Vf = 1,1V Mini - 1,5V maxi

Then you may use these conditions with :
(5 - 1,1)/0.005 = 780 Ohms

But if you read all the datasheet, you can see that it works even with lower If values , then R=1K is OK too
You could use lower resistor value, as long as you stay in the 30mA (safe or arduino output, and safe for the opto too : 60mA maxi), no need to use much current if it works with less
I would go for any resistor between 760 Ohm and 1,5K

Look in "ELECTRICAL CHARACTERISTICS"

Forward voltage of the "INPUT" is 1.1V at 5mA. That's the LED inside the chip.

Your resistor therefore needs to drop 3.9V at 5mA. Use Ohm's law.

Thank you both for your replies. That makes sense just each data sheet seems to show things in different ways so its really understanding where to look as there are lots of numbers that can confuse us newbs.

So looking at another data sheet for a 6N137 that I also have looks like I would use a 360 Ohm resistor based on the following...

Test Condition If = 10mA
Vf = 1.1Min - 1.7 Max with a typical of 1.4

So 5v-1.4v = 3.6 x 0.01 (10mA) = 360 Ohms

Is that correct?

Yes, but it is not "x" , it is "/" : R=U/I

Test conditions gives you the conditions they use to make the measurements, not what you must do.
In that datasheet, ther is another section :
Recommended Operating Conditions

The Recommended Operating Conditions table defines the conditions for actual device operation. Recommended
operating conditions are specified to ensure optimal performance to the datasheet specifications. Fairchild does not
recommend exceeding them or designing to absolute maximum ratings

Then try to stay within these conditions.... and it is OK if If=10mA 8)

Whoops big typo mistake of 'x' instead of '/', will double check before I post again.

Once I can fully understand the data sheets then hopefully will get more confident, but now I have a good resistor value for my optocoupler then will build this circuit up and give it a whirl!

Thanks again

Just thought I would post an update to say that I got the flash trigger working perfectly so thanks for the help. Is there a projects area where people post up project detail as would like to share the a bed project that I completed.

One simple point - you can measure the forward voltage roughly by choosing a conservatively
valued resistor (say 1k for 5mA), and applying your multimeter to the optocoupler emitter pins...