Optocoupler flash trigger


I had this old flash lying around and I thought I would try to hook it up to an arduino. A bit of googling told me that an optocouper would be the way to go to shield the arduino from the high voltages.
Everything seemed simple enough, but I just can't get it to work.

I opened the flash and hooked wires up to the flash test button. Removing the little button even gave a nice hole for the wires to go through. Touch the wires together and the flash goes of.
I used a 4N35 optocoupler, a PS4N35V to be exact. I hooked a digital pin up to pin 1 on the optocoupler through a 220 ohm resistor. Pin 2 to ground. I hooked the flash wires up to pin 4 and 5. I'm guessing polarity isn't an issue here since I'm just closing a circuit.

I then wrote a super simple test code writing the digital pin plus the pin 13 led high for a second then low for 10. Led is flashing but the flash isn't going off.
I've looked at a couple of other projects and it appears I'm doing the right thing. Am I missing something obvious here?
I tried putting an led with an external power supply in instead of the flash and that works fine. It flashes in sync with the onboard led.

One thing I've noticed is that when I connect the flash to the optocoupler it goes off. I thought that pin 4 and 5 wouldn't be connected unless there was current through 1 and 2. I must be misunderstanding something.

Any advice would be very much appreciated.

You've made no mention of what voltages and currents are involved. For example, you probably wouldn't imagine turning on a washing machine's main power through the optocoupler, right?

Polarity matters with an optocoupler. It is not just closing a circuit. The optocoupler contains a transistor.

Voltage matters also.

You should not have been connecting things with the power applied.

I'm guessing polarity isn't an issue here since I'm just closing a circuit.

A transistor (or phototransistor) isn't a relay or switch* and polarity does matter and we don't know the polarity of what's coming-out of the flash unit. Try reversing those connections.

We don't know the polarity, voltage, or current through the flash trigger switch so you're kinda' shooting in the dark.

  • We often talk about a "transistor used as a switch" and it can switch on/off voltage/current, but it's not always a drop-in replacement for a regular open-close mechanical switch or relay.

I mean that the polarity doesn’t matter to the flash. I’m not powering the flash through the optocoupler. I’m closing a switch in the flash that tells it to go off. I’ve also tried switching the wires and it makes no difference. What voltage should I be measuring?
I’m doing the same as in this project.

Yes, polarity makes a difference.... as well as current. I am surprised that the flash you see, when you hook everything up, isn't the poor little opto going poof in the night...

try measuring the voltage level and polarity on the two wires with open circuit, then the current (start in AMPs) when the circuit is closed. then look up and READ the datasheet for the opto.

Ok. I see there are some basic things I'm not understanding about this. I tried measuring the voltage on the wires from the flash, found the polarity and there's 130 volt on it. A wonder I haven't fried anything yet.

If there is 130 volts to control, a 4N35 is the wrong thing to use and yours is probably dead.

I'm think I'm getting what my issue is here. There's some basic stuff I didn't understand about what I was doing. And not all flashes are built the same. Just to make sure I'm not taking a wrong turn here, is it the collector-emitter breakdown voltage of the optocoupler I need to get right? Needs to be more than 130v?

No, but at what current?

I'm not powering the flash through the optocoupler. I'm closing a switch in the flash that tells it to go off.

And then you say you clocked 130V somewhere?
No offense meant, but I wouldn't trust your conclusions if I were you.

Go back and make sure your optocoupler is working the way it should. Run a simple battery + led on the output side. A bare led and a coin cell is as simple as it gets. Or measure continuity.

Direction matters on both sides of the opto.

130 sounds right, depending on which side of the trigger transformer he’s on.

No offence taken. In the comment about the the measurement I also mention that I realize I've misunderstood some things. 130v going through what I thought was a low voltage circuit was one of these things. Probably didn't make that clear enough. One comment is before learning something new, the other is after. You don't know what you don't know. Now I know.

I believe the conclusion is I need a different optocoupler. I'm just a bit unsure which rating I'm looking for in the datasheet.

And I'm also not really sure how to go about measuring the current of the flash.

If your multimeter has a min/max mode, that'd do it.

It doesn't appear to have that. A thing to keep in mind when the time comes for an upgrade. It can tell me the humidity and light level if that gets me any further.
I've opted for a mechanical solution until I have learned more. If I'm understanding this, and please do correct me if I'm wrong, I need an optocoupler that can handle a collector-emitter voltage higher than 130v. And probably a collector current of a couple of amp.

The "safest" thing to replace a switch in an unknown circuit would be a relay. A relay is a an electro-magnetically operated switch.* Relays rated for more than 100V (and more than 1A) are common.

Relay contacts are isolated and they are "electrically rugged"... That is, if you go a little over the rated voltage or current they usually don't instantly burn-up but you might get shorter life out of it.

You will need a driver circuit (you can't drive a relay coil directly from the Arduino) or you can get a relay board that has a relay and a driver.

  • There are solid-state relays but they are not always a replacement for a mechanical switch and you need the right one for the particular application.

I actually ordered a couple of theselast week. But as far as I can see they can only handle 30v. Just have to find the right one.

Or use a simple reed relay and a diode cross the coil; Arduino can drive this one directly.

Do you want to 1/ trigger the flashgun or 2/ detect when it has been triggered?

1/ an opto triac could be the best approach. A thyristor is generally used to trigger the tube with a high voltage (10kV? ) stepup transformer , and in more modern devices a IGBT is used to turn off the tube current. I've designed them.


2/ Why mess about about with dubious electrical connection to your flashgun? - unknown polarity , probably high voltage + very fast transients...... Use a fast phototransistor to pick up the light.


It is the first option. An opto triac is what I'm looking at now. Glad to know I'm not going the wrong way. Until I get my hands on some I just have a servo touching the two wires together. Works great.