[sdiy] opto isolator "Vactrol" tester

[Oren Leavitt]

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
>