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

[poldervaart]

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.youtube.com/watch?v=R8_dAgaBBdI
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1208117338
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1264441299

[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.

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.

[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.

[poldervaart]

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. 

[poldervaart]

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?

[poldervaart]

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

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

[wanderson]

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.

[dc42]

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.

[wanderson]

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

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?

[poldervaart]

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

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?

[wanderson]

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...

[poldervaart]

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. ;-)

[wanderson]

To measure the flash current you need an oscilliscope with storage

[poldervaart]

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.

[poldervaart]

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?