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Topic: Optocouplers (Read 984 times) previous topic - next topic

neema_t

Hello everyone, I need some help!

I have two projects that involve optocouplers in the planning stages at the moment, but I have no idea what they're about as I'm still really new to electronics.

One project is a camera shutter/flash trigger, so I want to use the couplers to isolate the Arduino from both the camera and flash to avoid damaging any of the three parts. The other project involves controlling between five and 16 buttons on a Playstation 3 steering wheel (I'm pretty sure I only really want 9, though).

The problem I'm having is that I don't understand the different types of optocouplers. RS Components has 981 Optocouplers with output types such as AC, DC, differential, digital, open collector, open drain, pull up, push-pull and three state; output devices include Darlington, darlington with base, linear photovoltaic, photologic...

I don't understand any of it! All I want is a straightforward connection to be made when the LED inside lights up, kind of like an optical version of a reed switch I suppose, mainly because I don't want to modify the signals that the camera or the PS3 wheel would usually send and receive without any additional circuitry. Can someone please explain to me what I should be searching for? I suppose it doesn't help that I don't understand what a MOSFET is or transistors at all in general.

Also ideally I'd like as little delay as possible, I read that some solid state relays (which these are, right?) have significant delay but as they're going to be used for controlling some time-sensitive equipment (the camera and flash, for example) the less delay between the LED illuminating and the connection being made on the other side, the better, given that the Arduino might be used to trigger a flash when a loud sound, such as a balloon popping, is 'heard'. Not much point if the flash goes off even as much as .25sec afterwards!

Thank you,
Neema

Grumpy_Mike

First read this:-
http://en.wikipedia.org/wiki/Opto-isolator

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I suppose it doesn't help that I don't understand what a MOSFET is or transistors at all in general.

Yep that makes it virtually impossible.

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I read that some solid state relays (which these are, right?)

Wrong a SSR might have an opto isolator on the front end but there is a lot more to SSRs than just the isolator.

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I read that some solid state relays .. have significant delay

Not really the delay in the button being pressed and the shutter going off in a digital camera is several orders of magnitude larger than any delay in an opto isolator.

Go for the cheapest mosfet output opto isolator you can find. A mosfet output is more likely to work with a camera than a transistor output.

neema_t

Thanks Mike, I've been reading up on optocouplers, SSRs, MOSFETs and transistors since my last post and to be honest, haven't really followed it. I have learned that transistors are used for amplifying the current through them and MOSFETs are most often used to convert voltages, which concerns me slightly as I only want to emulate the operation of a switch with my arduino, and switches don't amplify the current nor do they alter the voltage passing through them very much, so I'm a bit apprehensive to continue with either of these projects until I've learned more.

I did, however, find a YouTube video tutorial on how to make an arduino-controlled camera and flash trigger, and the guy there used an AQW280EH solid state relay (which he confusingly referred to as an optocoupler). I can only find it at mouser.com for a fairly hefty £2.64 (considering I'll need at least 11 of them) per chip and an even heftier £12 shipping charge as it's coming from the States! However, I've been looking at other photoMOS solid state relays and can't see anything in the datasheet to specifically suggest they alter the current or voltage passing through the output side other than through normal resistance and such. So, how about a photoMOS relay, as expensive as they are? I might pick one up and have a play with it, unless that's a Very Bad Idea?

Thanks again!

Grumpy_Mike

You don't need an SSR to trigger a flash, you just need an opto isolator. A mosfet changes its resistance from very high to very low, that is what you want to do. A flash gun has a high voltage on the switch contacts maybe 600V or more so it is important to get an opto with an output FET that can handle this sort of voltage. Don't be confused by the isolation voltage it is not the same thing as the breakdown voltage of the output.
You can get the AQW280EH from the UK but the postage might be high:-
http://uk.farnell.com/panasonic-ew/aqw280eh/photomos-relay-1500v-120ma/dp/1853864?Ntt=AQW280EH
maybe you can get other stuff at the same time or search the site for something similar.
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for a fairly hefty £2.64

That is a good price for something with that spec.

neema_t


You don't need an SSR to trigger a flash, you just need an opto isolator. A mosfet changes its resistance from very high to very low, that is what you want to do. A flash gun has a high voltage on the switch contacts maybe 600V or more so it is important to get an opto with an output FET that can handle this sort of voltage. Don't be confused by the isolation voltage it is not the same thing as the breakdown voltage of the output.
You can get the AQW280EH from the UK but the postage might be high:-
http://uk.farnell.com/panasonic-ew/aqw280eh/photomos-relay-1500v-120ma/dp/1853864?Ntt=AQW280EH
maybe you can get other stuff at the same time or search the site for something similar.
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for a fairly hefty £2.64

That is a good price for something with that spec.


Ah great, thank you, I didn't even think to check Farnell, but their prices are pretty much the same as Mouser's. I think I'll see what kind of PhotoMOSs RS Components have in stock and try to find something similar to the AQW280EH maybe.

Do you mind me asking what the isolation voltage and breakdown voltage are? I'm guessing the isolation voltage isn't something I have to pay much attention to right now, given that the AQW280EH's is 5,000V. Is the breakdown voltage the same as the load voltage? My flashgun's trigger voltage is between 220-230V so the AQW280EH's load voltage of 350V should handle that fine.

Thanks again, Mike, you're a great help!

Grumpy_Mike

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Is the breakdown voltage the same as the load voltage?

Sort of it is the maximum load voltage you can have. This should be at least 80% higher than the voltage you want to switch.
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Farnell, but their prices are pretty much the same as Mouser's

Mouser have a very large minimum order or else a large postage component.

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