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DIY opto isolator

:: EPE Chat Zone ­:: ­Radio Bygones Message Board :: » EPE Forum Archives 2005-2006 » Archive through 07 November, 2005 » DIY opto isolator « Previous Next »

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Terry
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Username: Terry

Post Number: 64
Registered: 05-2005

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Posted on Sunday, 23 October, 2005 - 07:16 pm:   Edit Post Delete Post Print Post

I want to make an opto isolater to fit between an R/C receiver and speed controller but not using a ready made package as the separation is not enough. My qustion is what is the smallest, lightest pair of devices I can use ? My intended air gap will be 20mm.

Thanks, Terry
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Microguy
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Post Number: 329
Registered: 09-2005

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Posted on Sunday, 23 October, 2005 - 08:14 pm:   Edit Post Delete Post Print Post

A small LED, like surface mount, and cadimum sulfide (?) photosensor.

Couple them together with some heat shrink tubing.

The photocell is pretty small and light to start with, and well, a surface mount LED or even just a tiny "regular" LED is small and light.

I could take some pictures and show you if you like.

It's a bit better if you have some stiff tubing, like that tubing with the "glue" inside of it, polyelfin I think it is. It melts when you heat it up, and it's fairly stiff so it holds it's shape well.
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Terry
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Post Number: 65
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Posted on Sunday, 23 October, 2005 - 08:36 pm:   Edit Post Delete Post Print Post

Yes you got the idea, I have done it in the past with 5mm devices but I want it a lot smaller and lighter. I can find 3mm devices Im sure but when it comes to anything smaller Im not sure which to use. I was hoping someone had used somthing they could recomend and a UK supplier.

Terry
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Mikehibbett
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Post Number: 239
Registered: 04-2005

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Posted on Sunday, 23 October, 2005 - 09:28 pm:   Edit Post Delete Post Print Post

An SMD led will also function as a light detector, so you could use two sot23 packaged LED's, one to transmit, one to receive. There was an article recently in EPE about using an LED as a detector. A google search will probably help.

Farnell sell SMD led's, but they have a £20 MOQ if you want free delivery charge. Alternatively I think crownhill associates are sellign some...

yep, here you go
http://www.crownhill.co.uk/product.php?prod=748

not sure of their post & packaging costs, but I think it's reasonable.

Mike.
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www.drivesentinel.co.uk - Home build GPS Speed Camera Detectors
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Microguy
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Post Number: 331
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Posted on Sunday, 23 October, 2005 - 10:18 pm:   Edit Post Delete Post Print Post

That's true Mike, I had forgotten about that. I remember Forrest M. Mimms III talking about that in a couple of his electronics manuals.

So if he REALLY wanted to go tiny, use two sorface mount LED's, right smack face to face.


Now THAT'S about as small as you're going to get!
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Terry
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Post Number: 66
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Posted on Monday, 24 October, 2005 - 04:41 pm:   Edit Post Delete Post Print Post

Thanks for the info Mike and the link too. I will give it a try, Im amazed I didnt know that.

Terry
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Terry
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Post Number: 67
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Posted on Monday, 24 October, 2005 - 07:34 pm:   Edit Post Delete Post Print Post

Is it just smd leds that will do this trick ?

Terry
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Bluebottle
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Post Number: 27
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Posted on Monday, 24 October, 2005 - 08:37 pm:   Edit Post Delete Post Print Post

Hi Terry as far as I know any LED will do it but some better than others but because they weren't designed to do it not much data is available so it is a case of experimenting HTH
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Atferrari
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Post Number: 104
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Posted on Monday, 24 October, 2005 - 08:41 pm:   Edit Post Delete Post Print Post

I recall doing this just for fun: carefully filed the top of a small transistor (whilst kept upside down to avoid any metalic particles dropping inside).

Naturally, you will tend to file the edges almost all around what is good. It will allow to take out the "cover", flat and almost totally loose.

Later, I took out, VERY CAREFULLY, most of the silicon grease (?) covering the die. Then a small LED was inserted and fixed somehow, quite close to it.

It worked + or - as expected.

The transistor still around on my bench after 5++ years. If I recall properly, a 2N222 or BC107, or perhaps an old (really old) digital one (?!).

I should check tomorrow when back at my office.

Without the LED also used to measure the frequency of the fan in the bench, with the blades interrupting the incident light.

Encouraged by that I did it with a 2N3055. This last, probably too big and too power hungry for your application.

Who knows, perhaps...or
Agustín Tomás
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Terry
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Post Number: 68
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Posted on Monday, 24 October, 2005 - 08:50 pm:   Edit Post Delete Post Print Post

I did it with a BC109 after reading it in EE in the 70's. There was no grease in them then. I just tryed it with a 5mm led but I cant get it to work so I will have to get some SMD ones or just use a normal 3mm IR pair.

Terry
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Microguy
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Post Number: 334
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Posted on Monday, 24 October, 2005 - 10:04 pm:   Edit Post Delete Post Print Post

Yes, just about any LED will work.

A long time ago, I drilled a tiny hole down into a LED, and stuffed a bit of fiber optic down there.

Did that for another LED and stuffed the other end down that hole.

So I ended up with a mini fiber optic link! Sure it's not optimal, or even good for that matter. but it works.

Of course, if you're not careful you'll drill too far and cut the very tiny wire down there.

I used to do a lot of stuff like that.

As for sawing off the top of transistors, I never did that. I'm surprised! I mean, I cut them up, but never powered it back up with it exposed.

Actually, I think that's sort of how the LED was discovered!

The were working on transistors when they noticed that the PN junctions were glowing a little bit. After a bit of looking and tweaking the doping process, Poof! LED's!!

Wish I could remember where I read that. So, my guess is, if you cut the top off that transistor again, and actually used it as designed, you'd see it glow just a little bit. You may need a magnifier to see it though. I think I'll try that when I get back.

Oh yeah, they did use a sort of grease inside of most of them, it helped dampen the vibrations some so those tiny wires woulndn't break lose. They don't all use it, and I'm not sure they still do. But I remember seeing a lot of it in my early days.

(Message edited by microguy on 24 October, 2005)
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Paul_goodson
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Post Number: 140
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Posted on Monday, 24 October, 2005 - 10:12 pm:   Edit Post Delete Post Print Post

Silly question but would a ready made package work if one was cut it in half and opened up to gain the 20mm gap
Would this be too heavy?
Also would it be as easy to use a surface mounted photo diode with the surface mounted led
Paul
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Bluebottle
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Post Number: 28
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Posted on Monday, 24 October, 2005 - 10:17 pm:   Edit Post Delete Post Print Post

They started putting grease in OC71 germamium transistors in the 1960's to stop people using them as photo transistors (OCP71) by scraping the paint off.
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Mikehibbett
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Posted on Monday, 24 October, 2005 - 10:22 pm:   Edit Post Delete Post Print Post

Talking about opening things up reminds me of some experiments we did in the early 1980's at university. We machined the tops of 1KB SRAMs (might have been DRAMs) to make very simple cameras. worked ok as I remember. Took some practice, destroyed lots of chips :o)
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Microguy
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Post Number: 336
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Posted on Tuesday, 25 October, 2005 - 12:32 am:   Edit Post Delete Post Print Post

I would think the dynamic RAMs would have been more sensitive to light as that's basically small capacitor that can't even hold it's own charge (why they have to be refreshed in the first place).

But then the question comes to mind, if you you erase what the light would have impressed on the circuits?

Interesting....

I always wanted to build a small photo transistor array, sort of a digital camera. I thought about using those very tiny PT's that are not much bigger than a pin head. They used them in paper tape readers. A lot of people are not aware that the basic CCD sensors are sensitive to infrared light.

But never got around to it, too expensive too. Man, the things that I either did, or wanted to do. Older and smarter now, I'd never do some of them. But you do learn a lot doing that kind of stuff. That's the basic stuff you learn, that you'll never forget.

(now why is refresh blanked out??, Ah, I get it, it's just refresh, and IT is the next word, so put those four letters together... man, that's a little sensitive eh?)
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Microguy
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Post Number: 337
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Posted on Tuesday, 25 October, 2005 - 12:34 am:   Edit Post Delete Post Print Post

Terry, one question comes to mind.

How in the world are you getting that high of a voltage that you need to make your own opto isolater??

Where are those kinds of voltages coming from in a RC airplane??

Or did I mis-understand your original post?
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Terry
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Post Number: 69
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Posted on Tuesday, 25 October, 2005 - 08:08 am:   Edit Post Delete Post Print Post

My plan is to opto isolate the reciever completely if I can build light enough. The interfierence comes from the long servo leads but mainly the 250W blushless motors. I fly the aircraft at the edge of its range and I want to increase this to the max.

Terry
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Atferrari
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Post Number: 105
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Posted on Tuesday, 25 October, 2005 - 10:12 am:   Edit Post Delete Post Print Post

Hola Terry,

What is the maximum Vdd available in that plane?

250W motors? Lot of power, ins't it?

Have you tried the trick with the transistor + LED using what circuit?

Interested in all this.

They started putting grease in OC71 germamium transistors in the 1960's to stop people using them as photo transistors (OCP71) by scraping the paint off.

Yes, my brother demonstrated that to me when I had no idea what a transistor was. I still have lot of them from one of my first projects... that was never built. (Shame to me, I can not recall a single detail of it!)
Agustín Tomás
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Pat
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Posted on Tuesday, 25 October, 2005 - 08:43 pm:   Edit Post Delete Post Print Post

I have over 1000 of those black painted OC71 germanium transistors in my workshop none of which I recall had any gell in them when I last used one over 10 years ago. I scraped the paint off about 50 of them and started to grade them according to their beta gain which varied from 20 to over 100. However although sensitive to ordinary light they are not sensitive to infra-red.

Pat
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Terry
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Post Number: 70
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Posted on Tuesday, 25 October, 2005 - 08:53 pm:   Edit Post Delete Post Print Post

Hi Atferrari

The max power will not be used for long, mostly the motors will be at cruise. Power is from 3xLi Poly cells which are rated at 11.1v.
'Have you tried the trick with the transistor + LED using what circuit?' I have only played with a converted transistor when I was young, I never used it for a serious circuit.
I seem to remember those OC71's were great for small radio projects.

Terry
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Poriet
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Post Number: 33
Registered: 06-2005

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Posted on Tuesday, 01 November, 2005 - 06:52 pm:   Edit Post Delete Post Print Post

Hi,
Any transistor chip is light sensitive ( or, to be pedantic, the PN junction is ) In 'Art of Electronics' they describe how this effect caused unexplained circuit malfucntions in the early days.
The old germanium transistors WILL make photo-sensitive devices, but the gain is lousy and the device fails before long. Silicon devices are MUCH better. Of course, most transistors you get nowadays are plastic, but I have a stash of elderly metal cans ; TO-5 and TO-66. You just have to saw the lid of the can off.
The TO-66 is fantastic ; it's VERY sensitive
( big die-chip ) and has the advantage that the casing has holes so you can bolt it down. If you use the TO-5 can ( still a very good photo-device ) it's tricky to actually fix it where it can do some good.
One more thing. The metal can is hermetically sealed ( no air ) I always assumed that if you cut it open the silicon device would either be dead or fail very soon. Not so. I built Andy Flind's 1995 automatic curtain-closing circuit.
My own version uses two TO-66 cans with the lids removed. They have been working faultlessly for 10 years even mounted on the outside of the house.

As to the original question: If you get a old
cheap single-click mouse ( scrap these days ) and rip it open, you will find TWO pairs of photo-diode emitter and phototransitor/photodiode
receiver. The devices are perfect for your application ; they are small and manufactured to look sideways when mounted on the board. And they are small. Worth a look, I think.
HTH
P.
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Mike_b
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Post Number: 79
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Posted on Tuesday, 01 November, 2005 - 09:36 pm:   Edit Post Delete Post Print Post

Ahhh ... scraping the coating off of OC71 transistors ... where the hell did all the time go ........

sorry ... day dreaming ....

I dont quite understand why you are going for the opto solution, but if you dont get enough power, how about "connecting" them with a optical fibre (pinch one off the Christmas Tree this year when everyone is sleeping off lunch).

rgds

mb
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Terry
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Post Number: 73
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Posted on Wednesday, 02 November, 2005 - 09:17 am:   Edit Post Delete Post Print Post

Mike, the opto solution is the only one that works 100% and is tried and tested. All the talk about converting transistors is all very interesting but no use to me as seven of them with led's to switch them will be far too big and heavy. The idea of using smd led's for both sides is good but I can't get it to work with standard units so I not ordered the smd led's.

Terry
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Mike_b
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Posted on Wednesday, 02 November, 2005 - 02:40 pm:   Edit Post Delete Post Print Post

Hi Terry,

I might be on the wrong track here as you have much more experience than me, but perhaps its worth tackling the interference problem and then investing the weight saved in getting the controller.

1. Encase the servo leads in silver foil taken to earth.
2. Place 100uH coil, with a decoupling capacitior in the +VE supply lead, both at the battery and the rx.
3. If you have a scope, play around unitl you see a drop in the noise.

Do they make a controller based on SMD power transistors, mounted with perhaps a duct, to capture air movement from the propellors wash.

Just ideas anyway ...

rgds

mb
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Terry
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Post Number: 74
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Posted on Wednesday, 02 November, 2005 - 05:21 pm:   Edit Post Delete Post Print Post

Thanks mike but like I said the opto solution is the only one that works 100% and is tried and tested. The normal ways of shielding and choking do work up to a point but it all tends to get long winded and messy. Some speed controllers do have built in opto isolators but they are not as effective as having a bigger gap in a separate unit. I just want to make the separate unit as light as possable.
'Do they make a controller based on SMD power transistors, mounted with perhaps a duct, to capture air movement from the propellors wash.' Not that I know of, modern controllers are very small and handle a lot of power. I did poke the heat sink out the side of the plane on a diy controller about 10 years ago to help cool it.

Terry

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