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Topic: Thanks! (added photos) :: Opto-Isolator Question for Camera Flash Trigger Proj (Read 10705 times) previous topic - next topic

dc42


In fact, based on this, if I go even to a 70ohm resistor, I will be down to the minimum voltage levels.  It would appear to me that even a 100ohm resistor would remove too much and would not trip the optoisolator?  Is this right?  It looks that way on paper, but doesn't "feel" right.


60mA is the maximum forward current rating of the optoisolator. It will work at much lower currents, depending on its current transfer ratio and how much current the flash unit sources. The Arduino output pins are rated at only 40mA. I would design for 10mA (if that is enough to trigger the flash) or 20mA.
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poldervaart


60mA is the maximum forward current rating of the optoisolator. It will work at much lower currents, depending on its current transfer ration and how much current the flash unit sources. The Arduino output pins are rated at only 40mA. I would design for 10mA (if that is enough to trigger the flash) or 20mA.


Ahh.. yes.  This makes sense.  So when I calculate Ohm's Law, It seems that if the item I need to power has a lower mA rating than the Arduino output pins, then I would use that for a max number in the current piece of the calculation, but in cases like this where the Arduino output has the lower value, I should calculate with that figure as the max current...

Even if I just keep everything at max, going off the 40mA change that brings me to a likely resistor rating of 100 Ohm (jumping to the next highest standard value).  Designing for 20mA puts it at 180 Ohm.  These values are much more in line with what I am used to seeing in schematics.

I may be getting too deep here, but what's the way to tell how low is too low?  My assumption is the only point of the output from the Arduino is the light the LED within in the optoisolator so the sensor within is able to recognize the condition and then close the other side of the circuit. Right?  I guess I don't understand how the flash values affect this.


dc42


I may be getting too deep here, but what's the way to tell how low is too low?  My assumption is the only point of the output from the Arduino is the light the LED within in the optoisolator so the sensor within is able to recognize the condition and then close the other side of the circuit. Right?  I guess I don't understand how the flash values affect this.


It appears to me that Canon doesn't publish the current that the driving device needs to sink when setting off the flash. So all I can suggest is to experiment. Vary the series resistor until you find the highest value that reliably sets off the flash, then work out the corresponding current. Choose a series resistor that provides at least double that current. If that means exceeding the 40mA rating of the Arduino pin, then choose a different opto isolator with a higher current transfer ratio (e.g. Darlington output) and start again.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

wanderson

Even 1ma of current through the LED would be enough for most purposes...  Generally use resistors in the 220 to 470 ohm range...

For the flash, the current is provided by the flash unit, not the circuit triggering the flash.  And while the instantaneous current is fairly high, it is also of a very short duration.  On the order of less than a millisecond at maximum power and microseconds at lower power levels. And the current is dictated by the flash power being used, which is one reason you will not see it documented.  The flash interface was created decades ago, and was designed to be simple and easy to trigger, originally with a mechanical switch--- Indeed I still own camera lenses with just such a mechanical switch.  And there physical size (very small conductors) would tend to confirm the low power handling ability needed.

Even the lowest level OPTO can handle the highest flash power setting for the frequency that such a device can handle (typically twice per second at maximum).  And there should be no resistor on the output of the OPTO that connects it to the flash.  The flash unit expects a simple contact, even a switch has been used for triggering the flash.  The resistor is only for controlling the current through the LED portion of the opto.

You would only need to start worrying when dealing with studio flashes, and likely not even then, depending upon what triggering devices they were designed for, since the industry tends to use fairly consistent devices.  The real problem with studio flashes is again not the current from a single flash, but rather the much higher flash frequency possible. So the power dissipation of the OPTO would be the big issue.

AWOL

Use a MOC3020 optotriac.
Six pin DIL package, 7.5kV of isolation, you drive it like a LED (10 to 15mA), switches up to 400V, no polarity issues on the output.
You can trigger them with pulses of a microsecond or less.

dc42


Use a MOC3020 optotriac.
Six pin DIL package, 7.5kV of isolation, you drive it like a LED (10 to 15mA), switches up to 400V, no polarity issues on the output.
You can trigger them with pulses of a microsecond or less.


Isn't there a risk that the triac would stay on once fired? Depending on how much current the flash unit sources.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

AWOL

It hasn't caused me a problem, from decades-old high voltage Vivitar 283s, to modern, almost logic-level, Canon and Pentax units, plus some homebrew mongrels.

wanderson



Use a MOC3020 optotriac.
Six pin DIL package, 7.5kV of isolation, you drive it like a LED (10 to 15mA), switches up to 400V, no polarity issues on the output.
You can trigger them with pulses of a microsecond or less.


Isn't there a risk that the triac would stay on once fired? Depending on how much current the flash unit sources.


Yes, there is a risk.  Some flashes, notably canons have a tendency to keep the triac active.  opto-triacs are not needed when dealing with flashes intended for modern digital cameras which do not tolerate the high voltages used in older style flash units.

poldervaart

I went ahead and picked this one up today:


3041:  NPN Transistor Output

The electronics store provided me the following summary information for it:

Total Device Ratings:

  • Isolation Voltage Surge (Volts)  :  VISO  :  7500

  • Total Power (mW)  :  PT  :  250

  • DC Current Transfer Ratio (%)  :  CTR  :  100


Maximum LED Ratings:

  • Forward Current (mA)  :  IF  :  60

  • Forward Voltage (Volts)  :  VF  :  1.5

  • Reverse Voltage (Volts)  :  VR  :  6


Maximum Phototransistor Ratings

  • Collector to Base Voltage (Volts)  :  BVCBO  :  70

  • Collector to Emitter Voltage (Volts)  :  BVCEO  :  30

  • Collector Dark Current (nA)  :  IC  :  20



I'm still not 100% what all of these values represent, but I think I understand the ones relevant to my project.

I bought a variety pack of resistors...so I will next figure out which I will use. I figure I'll start high, and work my way down if I have issues.

I'll post results when I get it working, so someone else can use this as a reference if needed.


poldervaart

Oh...hey. While I still have your attention.  Can you help me determine the 1 pin on this?  I assume this might be common knowledge for most of you, but this actually has me stumped at the moment.

The data sheet and packaging for the item shows as follows -- with the dot as I expected for the 1 Pin location.



But as you can see from the photos of the item I have in hand...it has no such dot. Instead, it has one side sorta cutout along the entire edge.

Anyone know definitively where the 1 pin is on this one?





EDIT:  After viewing many images of optoisolators on the Interwebs, I get a strong sense that the cutout or beveled edge is the input or LED side where pins 1-3 are. I haven't seen any documentation that confirms this.  I haven't yet seen a non-dotted and beveled optoisolator that specifies the pinouts, but I have seen several beveled ones that also have dots, and the dot is always on the beveled side.  I'm making a leap here.


dc42

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poldervaart

I just wanted to report back and let you know I was able to successfully execute on my Anrduino based flash trigger project.  Here's a couple of photos that were a result of the help you have provided in answering my opto-isolator questions.

 






patrickc01

Great pictures, and the detail is awesome!  How did you get such precise images?  I am working on the same project, but all of my test shots end up with a slight blur if the object is in freefall.  If I move the object slowly in front of the laser, images are good.  Did you run into this problem?  I am guessing it must be that the flash is too long, but I am not sure how to shorten its duration.  (I am using a 430 EX II).  Any advice would be great! 

poldervaart

Thanks!

I use the 430 EX II as well.  The key here is, as you stated, flash duration.  You can control the duration by the "power" of the flash.  Put it into manual mode, and bump the power as low as you can. Start from there.  The power of the strobe = flash duration.  So in order to freeze the moment, you want the shortest duration (or the lowest power) that will work.  I think I was in the 1/64 range for these shots.  Use ISO if you need "more light".  I was running ISO 320 and f/5.0.  

Good luck!  It was a fun project.  I also built a version that would detect sound and did some balloon pops. Equally fun!  : )

By the way..I liked the laser trigger so much I basically made a permanent add-on for my arduino.  Pics attached.  Basically I built the laser and photosensor into the "box" and use a mirror to bounce the beam back.  This gives me 2 beams on the same plane...and makes it easier to trip.  Also...have some different lights for ready state, etc.


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