Thanks! (added photos) :: Opto-Isolator Question for Camera Flash Trigger Proj

Other Hardware General Electronics
1

Hello,

I am new to Arduino. Completely. I decided to take the plunge this weekend because I will be pulling an all-nighter and was looking for a project to work on during that time. An Arduino project sounded like fun.

Because I will be doing this over night, I need to try and round up as many components as I can ahead of time because I won't be able to run to the store if I need something else. The problem....I have no idea how to know what I need. That's what brings me here for your help.

I found several how-tos on building a sensor based camera flash trigger, and I decided this is where I will start. I have a Canon EX430 flash as well as an external studio strobe. I plan to start with the strobe and the PC Sync cable to trigger from the Arduino.

Everything I have seen on this type of project uses a Opto-Isolater to separate the sensor side of the circuit from the flash side. That makes sense. What I don't fully understand is how I know what specific Opto-Isolator I need and what resistors I need to go with it.

I have a local electronics supply store that has 5 model numbers of NTE Opto-Isolators in stock. How do I know which one I need?

Here are the spec sheets for the 5 versions. I honestly don't know what I should be looking at to decide between the 5.

Part 3041 http://www.nteinc.com/specs/3000to3099/pdf/nte3041.pdf
Part 3042 http://www.nteinc.com/specs/3000to3099/pdf/nte3042.pdf
Part 3043 http://www.nteinc.com/specs/3000to3099/pdf/nte3043.pdf
Part 3044 http://www.nteinc.com/specs/3000to3099/pdf/nte3044.pdf
Part 3045 http://www.nteinc.com/specs/3000to3099/pdf/nte3045.pdf

Of course if I figure out what opto-isolator I need, I still don't fully understand how to know what size resister I need.

This is like greek to me (today), so your help with this first project is greatly appreciated. I expect after this project I'll know a lot more about what is going on...but I just want to make sure I get a good start.

Here are some of the similar projects that I hope to imitate:

http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1208117338
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1264441299

2

You need to know 2 things for selecting the optocoupler.

  1. The maximum voltage you will be switching.
  2. The maximum current you will be switching.

Then you look in the data sheets, and find the optocoupler with a collector-emitter voltage and current that exceeds what you will be working with.

For the resistor? Well, treat the input side as a normal LED (which it is - just an infra-red one) and calculate the resistor accordingly. The data sheet has the forward voltage and forward current for the LED, so use those, along with ohm's law, to calculate a resistor.

3

The Canon 430EX are modern flashed intended for digital cameras. I believe the voltage across the sync terminals are 8-10v, but you should verify this with a multimeter. If so you really want a transistor output for the optioisolator. Earlier high voltage flashes (200-400V) would be better served by an optoisolator that uses SCR output. The above is not hard and fast as long as the output can handle the voltage and current being used.

Personally, for my Canon580EX I just use a 4066 switch to make the connections since I am not terribly concerned with the digital compatible voltage levels frying my arduinos.

4

majenko:
You need to know 2 things for selecting the optocoupler.

  1. The maximum voltage you will be switching.
  2. The maximum current you will be switching.

Then you look in the data sheets, and find the optocoupler with a collector-emitter voltage and current that exceeds what you will be working with.

While I don't yet know the maximum voltage and current, I was looking at the data sheets to see if I could find the differences. First of all, there seems to be 3 different types of Optocouplers here. There are only two options on max voltages (30V and 80V), and only one lists a continuous current on the output information.

3041: NPN Transistor Output

Collector–Emitter Voltage, VCEO: 30V
Continuous Collector Current, IC: 150mA

3042: NPN Transistor Output

Collector–Emitter Voltage, VCEO: 30V
Continuous Collector Current, IC: None listed

3043: NPN Transistor Output

Collector–Emitter Voltage, VCEO: 80V
Continuous Collector Current, IC: None listed

3044: NPN Darlington Transistor Output

Collector–Emitter Voltage, VCEO: 80V
Continuous Collector Current, IC: None listed

3045: Silicon NPN Darlington Phototransistor Output

Collector–Emitter Voltage, VCEO: 80V
Continuous Collector Current, IC: None listed

My assumption at this point is that I don't need a Darlington pair transister, and should focus on the first 3 options. Am I understanding correctly?

Am I looking at the right voltage figure on the data sheet? What about "collector current"?

Thanks! I appreciate the time. I want to understand why I need what...rather than just buying parts from someone else's parts list.

5

wanderson:
The Canon 430EX are modern flashed intended for digital cameras. I believe the voltage across the sync terminals are 8-10v, but you should verify this with a multimeter. If so you really want a transistor output for the optioisolator. Earlier high voltage flashes (200-400V) would be better served by an optoisolator that uses SCR output. The above is not hard and fast as long as the output can handle the voltage and current being used.

Personally, for my Canon580EX I just use a 4066 switch to make the connections since I am not terribly concerned with the digital compatible voltage levels frying my arduinos.

Thanks for the info.

I'll check the voltages tonight for the two flashes.

Based on the information I just posted, it looks like the first three options meet the criteria of "transistor output", correct?

6

...and if it's not clear yet, this is my first time to try and interpret a data sheet. :slight_smile:

So I am trying to figure out what I should be looking for, and then how to interpret that information to apply it.

7

Any of the first three should work fine. Connect the input (two pins) of the optoisolator up to an Arduino output like an LED (add a current limiting resistor 220-470 ohms) and the connect the flash sync pins to the two output terminals of the opto isolator.

8

First you need to establish the voltage and current you need to switch. Use a multimeter on the voltage setting to measure the voltage between the terminals of the flash unit. Then disconnect the multimeter, switch it to read current, and connect it again (which should set off the flash) to measure the current.

Choose an opto isolator with a collector-emitter voltage and collector current rating that is above those figures. Also, you want the required current divided by the current transfer ratio of the opto to be low enough for the Arduino to drive (aim for 20mA or less). In practice, this means you may need a Darlington output if the current you are switching is more than about 10mA,and a non-Darliongton output is probably better if it is less.

9

poldervaart:
...and if it's not clear yet, this is my first time to try and interpret a data sheet. :slight_smile:

So I am trying to figure out what I should be looking for, and then how to interpret that information to apply it.

The voltage you listed from the data sheets is the maximum voltage that is safe for the optoisolator to switch. The one component you need to evaluate that is the voltage placed across the sync pins of the flash you want to use. The Canon 430EX you want to use has about 8-10V across its sync pins (and the current pulse is brief and not very high), so any of the optos you listed can safely handle that.

When you select a transistor, you want it to have a voltage rating about 50% higher than the maximum voltage you want to switch. This margin is needed for a variety of reasons. Does that help explain?

10

wanderson:

poldervaart:
...and if it's not clear yet, this is my first time to try and interpret a data sheet. :slight_smile:

So I am trying to figure out what I should be looking for, and then how to interpret that information to apply it.

The voltage you listed from the data sheets is the maximum voltage that is safe for the optoisolator to switch. The one component you need to evaluate that is the voltage placed across the sync pins of the flash you want to use. The Canon 430EX you want to use has about 8-10V across its sync pins (and the current pulse is brief and not very high), so any of the optos you listed can safely handle that.

When you select a transistor, you want it to have a voltage rating about 50% higher than the maximum voltage you want to switch. This margin is needed for a variety of reasons. Does that help explain?

Very helpful, Yes!

So another question. Why does only one sheet list the Connector Current, if that seems to be such an important value? Or perhaps am I looking at the wrong data point for current?

11

poldervaart:
Very helpful, Yes!

So another question. Why does only one sheet list the Connector Current, if that seems to be such an important value? Or perhaps am I looking at the wrong data point for current?

Well, the datasheets you linked to are really 'summary sheets' so they do not provide complete specifications for the device. Also, optoisolators are generally just interface devices, so are not usually used to drive high current needs. Think of them as electrically isolated digital outputs like on the arduino (which can supply up to 40ma), for more than that you would use them to drive the base (or gate) of a transistor which would in turn handle the needed currents...

12

I measured the voltage of the two flash units I would be using.

Canon 430EXII: 9.5V
StrobeLite: 7.6V

I was unable to measure current thanks to a blown fuse in my multimeter.. Good news is that now I know that I need to replace it. :wink:

13

To measure the flash current you need an oscilliscope with storage

14

majenko:
For the resistor? Well, treat the input side as a normal LED (which it is - just an infra-red one) and calculate the resistor accordingly. The data sheet has the forward voltage and forward current for the LED, so use those, along with ohm's law, to calculate a resistor.

I hope you don't mind spending a little more time with me on this.

Based on the voltage readings from my flashes, any of the first three optoisolators I named would work...so lets just pick the first one as an example.

In order to figure out what resistor I need, I would use the following:

Continuous Forward Current = 60mA
Forward Voltage (Max) = 1.5V

If the output pin on the Arduino is 5V, then this should be my formula to calculate resistance using Ohm's Law:

R = ( V-Arduino – V-LED ) / LED-mA
R = (5 - 1.5) / .060
R = 3.5/.060
R = 58.33

Is my math right on this? Using the next highest resistor value should be what I want correct?

That seems a lot lower than what most people cite as a typical LED resistor value. What am I missing?

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.

Thanks again for the time.

15

wanderson:
To measure the flash current you need an oscilliscope with storage

Ha! I'm out, then. Can I assume the flash current is a non-issue now that I can confirm the voltage, or is it critical to measure?

16

poldervaart:
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.

17

dc42:
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.

18

poldervaart:
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.

19

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.

20

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.