[sdiy] opto isolator "Vactrol" tester

[Ben Stuyts]

Also cover them / shine a light on them. I once had a vactrol (NVOS) that behaved funny sometimes, so I replaced it with another one. Afterwards I found something that looked like a small crack near one of the leads, so I assume that some light leakage came in. (Or perhaps it was just moisture.)

Ben

> On 20 Oct 2025, at 01:07, crystal via Synth-diy <synth-diy at synth-diy.org> wrote:
> 
> Yes, that circuit sounds good. I think i can just go the breadboard, scope, meter route again. 
> 
> Indeed, some of the optos  took forever to get back to off voltage and the on and off timings were very inconsistent. It's not mission critical and i still have some separated (somewhere). I use them to modifry synth pcbs to add CV to circuits that didn't have those functions (i use OTAs also but optos are easy). I'm just digging my workshop out of a storage unit after a 9 year hibernation, planning to get back to synth building with state of the art, cutting edge (1987) tech.
> 
> thanks
> 
> On 10/19/2025 11:43 AM, Oren Leavitt via Synth-diy wrote:
>> Some years back, I made a Vactrol test circuit for quickly analyzing and 
>> matching Vactrols.
>> 
>> Basically a standard "cheap LFO" integrator/comparator w hysteresis 
>> oscillator, but using the Vactrol under test in place of the integrator.
>> 
>> The photocell side would go to the comparator input and the comparator 
>> output would drive the LED thru a current limit resistor. A series 
>> resistor from V+ with a pot is used to set a nominal current thru the 
>> photocell.
>> 
>> I can look at the frequency, shape and amplitude of the repeating 
>> "attack/decay" waveform on an oscilloscope as a quick analysis of the 
>> Vactrol.
>> 
>> -Oren
>> 
>> If I find or recreate the circuit, I'll post it here.
>> 
>> On 10/19/25 3:58 AM, crystal via Synth-diy 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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