Photo resistor output optocoupler

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zphere
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Posted by zphere: Tue. Aug 23, 2011 - 04:25 PM
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I am looking for an optocoupler with photoresistor output.

Extensive searching has turned up one series of parts from Silonex which would be acceptable, but Allied Electronics is the only distributor in the US. Element14/Farnell in Britain is a dirty, filthy joke.

When I ordered from Allied before, I created a custom email address just for them, and shortly after I placed the order, I started receiving spam from that address. I do not want to reward them with a second order after having misused and abused my email address.

I also found Tesla Blatna, but they only have a business-card website, and I presume will shortly be defunct due to lack of sales. :roll:

Digikey and Mouser have disappointed me with zero availability of resistor output optos.

Thank you for reading my post, and...

Thoughts?

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Learning and Information, General Electronics
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daqq
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Posted by daqq: Tue. Aug 23, 2011 - 04:55 PM
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You might have a problem there, since due to RoHS they aren't made any longer (atleast not that I know of). They contained Cadmium (CdS), which is a big no-no.

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Chuck-Rowst
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Posted by Chuck-Rowst: Tue. Aug 23, 2011 - 05:40 PM
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zphere,
What's you application?
Why do you need such an "odd-ball" part?

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Plons
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Posted by Plons: Tue. Aug 23, 2011 - 06:52 PM
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I needed such an optocoupler in a theremin design. An LED, an LDR and some shrinktubing. It worked fine.

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zphere
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Posted by zphere: Tue. Aug 23, 2011 - 07:03 PM
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daqq
Ouch. I had not thought of that. I noticed that CdS cells were getting scarce, but it did not occur to me that RoHS got them.

Chuck-Rowst
I am trying to replace a potentiometer in an application where I don't exactly/much/at-all know the waveform going into the ends of the pot. Is it AC? DC? What voltage? Changing voltage? The goal is to replace the pot with something I can control from an AVR.

Plons
I'd do that in a heart beat for a one-off, but I need something I can buy in production numbers and give to a board assembly contractor.

----
Thank you.

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RickB
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Posted by RickB: Tue. Aug 23, 2011 - 11:14 PM
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For production quantities you may have no choice but to use Allied. They seem to have cornered US distribution of Silonex and Excelitas(makers of the Vactec Vactrol optocouplers).
You might contact Excelitas to see if you can get parts from anyone else.

http://www.excelitas.com/ProductPages/Analog_Optoisolators_and_Optocouplers.aspx

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daqq
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Posted by daqq: Wed. Aug 24, 2011 - 07:35 AM
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Quote:
I am trying to replace a potentiometer in an application where I don't exactly/much/at-all know the waveform going into the ends of the pot. Is it AC? DC? What voltage? Changing voltage? The goal is to replace the pot with something I can control from an AVR.
There are digital potentiometers available from Maxim or other companies. Not exactly what you want, but still...

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Lennart
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Posted by Lennart: Wed. Aug 24, 2011 - 10:20 AM
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The LDR's I've seen accept pretty high voltage (250V) but not high current.

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Chuck-Rowst
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Posted by Chuck-Rowst: Wed. Aug 24, 2011 - 01:56 PM
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zphere,

Knowing only what you have told us about this application, my approach would be the following:

A. Determine if I can use a digital potentiometer in the target system. You will need 5v or 3.3v for the digitial pot on the application side, and the digital pot will likely need to share the ground of the target system as well. There are many choices for digital pots these days. Some may be more appropriate than others for the target system. I would take voltage and scope readings off of the existing target sytem pot you are looking to replace, then identify a candidate digital pot or two.

B. Connect the AVR to the digital pot control line(s) with plain-vanilla opto-isolators.

C. I've been waiting for analog optoisolators for about as long as I've been waiting for a schematic editor than can auto-route nets and allow me to use different colors for the nets. It doesn't seem like I'm going to see either anytime soon. My general design rule is to stay away from components with limited choices and availability.

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zphere
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Posted by zphere: Wed. Aug 24, 2011 - 02:07 PM
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RickB
Thanks for pointing out the Vactrol optocouplers; very interesting.

daqq
Thanks. I have looked at a few digi pots and the datasheets say that the voltage on the pot wires cannot exceed the power supply rails. The pot drive could very well exceed 5v. What I would really like is a mems pot, but it looks like they're still the things of IEEE papers behind paywalls, and not something I can get on the retail market.

Lennart
Thanks. I think that will be OK. The pots I'm replacing are all 10k or greater.

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zphere
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Posted by zphere: Wed. Aug 24, 2011 - 03:39 PM
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Chuck-Rowst
Thank you.

I will look into the digital pot again and see if i can find a suitable part. I had not thought of using an opto to isolate the supply voltages. I agree, the idea of designing around a part only available from a single manufacturer was not setting well with me.

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jgmdesign
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Posted by jgmdesign: Wed. Aug 24, 2011 - 06:41 PM
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Try something called a VACTROL. They work awesome!!

http://www.alliedelec.com/search/productdetail.aspx?SKU=9800705

Jim

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rogerray
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Posted by rogerray: Thu. Aug 25, 2011 - 06:06 AM
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I think linear photocouplers like PC817 will also works for your project. The current at the output side is propotion to the input current, linearity is much better than a VACTROL. It will save you a lot of $, $6.53 vs $0.2.
http://search.digikey.com/script...
http://www.siliconray.com/index....

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RickB
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Posted by RickB: Thu. Aug 25, 2011 - 03:22 PM
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rogerray:

Photo-transistor opto-couplers are not suitable for the intended purpose, i.e controlling audio signals with low distortion.

Are you connected with siliconray? You mention them in your 2 posts.

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zphere
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Posted by zphere: Thu. Aug 25, 2011 - 05:24 PM
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jgmdesign
Thank you for the link.

RickB
I'm reasonably confident I can correct predictable non-linearity in software. What I really need is to be able to pull the wiper line to both rails through a resistive connection rather than something with a fixed voltage drop (BJT). Since I am replacing multi-K pots, the input impedance of what ever is reading the wiper should be high enough that it won't draw much current, and then being a few ohms away from the rail won't be a problem. On what are originally 1k to 50k pots, it is only 0.3% error at most.

Using a MOSFET with a low Rds(ON) and a suitable ohmic region is one possible solution that I am investigating. It would be nice to find an analog fet output opto with low on resistance, but I'm not finding such an part. The Fairchild H11F1M would be nice, but has 200R on resistance.

Thank you.

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