[sdiy] Vactrol vs H11F1M optoFET

[Sean Ellis]

Fer has a filter built around these:
Https://doepfer.de/a1016.htm<https://doepfer.de/a1016.htm>

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From: Synth-diy <synth-diy-bounces at synth-diy.org> on behalf of mark verbos via Synth-diy <synth-diy at synth-diy.org>
Sent: Friday, 24 October 2025 10:26 AM
To: Mike Bryant <mbryant at futurehorizons.com>
Cc: synth-diy at synth-diy.org <synth-diy at synth-diy.org>
Subject: Re: [sdiy] Vactrol vs H11F1M optoFET

These are an interesting subject. I thought they might be a drop in replacement for a Vactrol, so I tried them out. I put 2 of them in a Sallen Key lowpass, like a lopass gate. The output from the filter was like the signal was going through a linear ramped integrator, rather than a filter. So, while it was filtering out the highs, it was not operating the same as a Vactrol would.  This is beyond my understanding, but I guess it is because the Vactrols in a lopass gate are simply resistors and have signal flowing in both directions where as this is more like the transistors in an MS-20 filter. However, in an MS-20 the signal is reduced way down before going through the transistors, where as in a lopass gate the signal is full level all the way through. So, in that respect it’s as if there is a comparator before the opto.

I imagine they would do just fine in more pedestrian applications.


Mark






On Oct 24, 2025, at 00:38, Mike Bryant <mbryant at futurehorizons.com> wrote:

On a related note, has anybody experience using these :

https://www.digikey.co.uk/en/products/detail/onsemi/H11F1M/1793945

Upsides and downsides ?
________________________________
From: Synth-diy <synth-diy-bounces at synth-diy.org<mailto:synth-diy-bounces at synth-diy.org>> on behalf of crystal via Synth-diy <synth-diy at synth-diy.org<mailto:synth-diy at synth-diy.org>>
Sent: 23 October 2025 22:31
To: mark verbos <markverbos at gmail.com<mailto:markverbos at gmail.com>>
Cc: synth-diy at synth-diy.org<mailto:synth-diy at synth-diy.org> <synth-diy at synth-diy.org<mailto:synth-diy at synth-diy.org>>
Subject: Re: [sdiy] opto isolator "Vactrol" tester

They're not labeled, but, i think they are that same part. I had some very mixed results , but, since i bought a large bag of them, i just threw the really slow parts in their own envelope. Since they were for my own circuits i liked the (occasional) lower on resistance.
Hadn't though to shine a light on them externally. Next time...
Crystal
On 10/20/2025 5:06 AM, mark verbos wrote:
I have had VERY unreliable experience with the Silonex parts. I used NSL-32SR3 which are meant to be the fast ones but I had places where they would take several seconds to decay. Oddly, it made a difference if I had them in an inline resistor verses as one part of a voltage divider. Also, not necessarily a bad thing, but their on resistance is much lower than in the Vactec/PerkinElmer/Excilitas ones.

Mark




On Oct 19, 2025, at 10:58, crystal via Synth-diy <synth-diy at synth-diy.org><mailto:synth-diy at synth-diy.org> wrote:

Can someone point to a testing circuit for "Vactrols?"
I bought a bag full of Siliconex optos years ago, they are all over the map in response time.
I had tested quite a few in a very crude breadboard setup, mostly just looking at on/off resistance timing and off resistance and then putting them in envelopes with details written on them. I'm sure i could do that again, i'm just wondering if it's been done better.

thanks,
 Crystal
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