Opto isolator datasheet help

Hey guys almost finished my laser triggered camera/flash project but the last step is getting my 4N25 optocoupler to trigger via a digital output.
I currently have;

output pin 13—>330ohm(5v/15mA=333ohm)—>opto anode…opto cathode—>arduino gnd…

9volt + —>470ohm—>opto collector…opto emitter—>red led anode—>red led cathode—>9volt -…

Having a hard time working out voltage drops and resistance required as I’m getting lost in the datasheet and not sure on the equations needed.

The 470ohm resistor is worked out only for supply voltage and led volt drop so … 9v-1.9v(led) = 8.1v/.015=540ohm…

Sorry for noob questions but i guess if i don’t ask I’ll stay dumb :slight_smile:


output pin 13--->330ohm(5v/15mA=333ohm)--->opto anode......opto cathode--->arduino gnd....

You're not getting as much current as you think. You have to account for the forward voltage drop of the opto LED (~1.3V). So you're getting (5V-1.3V)/330 = 11mA, probably a bit less as the Arduino will not put out exactly 5V at 11mA.

9volt + --->470ohm--->opto collector......opto emitter--->red led anode--->red led cathode--->9volt -....

It's the right idea but optocoupler transistors are not really meant for "high" currents. There should be enough current to light an LED, but I'd suggest another transistor output stage to make the optocoupler transistor work as little as possible (i.e., carry very small current).

I think you may be limited by the CTR (current transfer ratio) of the 4N25. It's a minimum of 20%, 50% typical. In the worst case (20%) you can only carry 20% of the LED current on the output side. Assuming you're truly getting 11mA flowing through the LED (remember...it's probably less) you're going to only get 20% * 11mA = 2mA allowed to flow on the output side.

-- The Gadget Shield: accelerometer, RGB LED, IR transmit/receive, speaker, microphone, light sensor, potentiometer, pushbuttons

Try putting the LED & resistor between the 9V and opto collector, with opto emitter to ground. 430 ohm should get you ~20mA thru the LED if the opto is turning on full.

See figure 14 in the datasheet, that's how NPN transistors are normally connected. http://www.vishay.com/docs/83725/4n25.pdf

Thanks for the quick response guys.
I’ts working with the led and resistor before the optocoupler cheers with 10mA to ground
Is there a need to still drop the resistance to the anode when it’s triggering at 11mA?

The LED in the this circuit is only a test as i wanted to use the opto-isolator as a switch for a camera flash bulb which i haven't been game to test the amps of.I know it has a 5v supply across the terminals when its ready to be fired but wasn't sure whether to use my multimeter across it to measure the current.

Also to use another transistor in the circuit for the flash could i hook the 2 flash wires to an npn transistor with the base going to the opto-isolator emitter?

Here are a couple of options, depending on whether you want the cameral voltage sitting High all the time, or Low, until ready to fire, taking into consideration that the Arduino turns on with I/O pins as inputs and floating high.