JonDent - Exploring Electronic Music: Vactrols

Sunday 3 December 2017

Vactrols - Optocoupliers

Vactrols and Optocoupliers

Vactrols are sometimes called Opto-isolators, photocouplers and optical isolators.

They all use a similar idea, though this post is mainly about vactrols.

One side has a a light source of some sort

This could be a LED (or an incandescent bulb).

The other side uses something which is photo sensitive.

This could be a LDR (Light dependent resistor) or a photosensitive transistor.

Applying a voltage to the LED causes it to light up. The light falls on the LDR.

The intensity of the LED directly controls the photo-transistor / LDR.

Thus this resistor becomes voltage controllable.

This is a Doepfer LPG... based on a Buchla 292 module.

It uses vactrols.

Through the use of a vactrol, anything which is normally controlled by a potentiometer or variable resistor can become a voltage-controlled parameter. Applying a voltage to the vactrol's LED is just like turning the knob on the potentiometer.

In addition, since the LED and LDR can be controlled by different circuits we are thus electrically isolating the two different circuits.

Thus, vactrols are sometimes called opto-isolators.

The hollowed space between the LED and photo-transistor/resistor can be made using glass, air, or a transparent plastic.

The resistance of the LDR doesn't change instantly.

It takes time ... the change is not linear either.

"It may take a few seconds until the LDR reaches it's dark resistance (i.e. the maximum resistance without illumination)" Doepfer.

A brief History

In 1967 Vactec introduced a compact RO (resistive optocouplier) branded as Vactrol. Unlike the tube-coupled ROs of Fender and Gibson, Vactrols were sealed and sturdy devices. They originally used incandescent bulbs. In the early 1970s, Vactec replaced the incandescent bulbs with LEDs. This increased the switching speed and probably also their reliability.

In 1983, Vactec was purchased by Perkins-Elmer. Their vactrol division was spun out from the company and changed their name to Excelitas in 2010.

In the European Union, the production and distribution of Cd-based photoresistors was banned after January 1, 2010. This decision marked the beginning of the end of Excelitas vactrols. However, CoolAudio started manufacturing the VTL5C3 and Xvive is manufacturing the complete range.

Commercial vactrols usually contain LEDs that usually produce infrared light.

The detector is a semiconductor-based photoresistor made of cadmium selenide (CdSe) or cadmium sulfide (CdS).

Resistive opto-isolators are the slowest type of opto-isolator .

Switching times usually exceed 1 ms, and for the lamp-based models can reach hundreds of milliseconds.

Cadmium-based photoresistors exhibit a "memory effect": their resistance depends on the illumination history; it also drifts during the illumination and stabilizes within hours.

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Since it's getting more difficult to find vactrols, making your own may one day be the only option.

These are the light dependent resistors which I like to use.

GL5549

The type of LDR is impt.

It should have a dark resistance of at least 0.5M ohm.

I like a slow attack (or response time).

VTL5C3s have a slope of 20, a dark resistance of 10M Ohms and a very slow response time.

LDR type nr 5516.are good too.

The 5516 specs say it gives about 5 to 10 kOhms at full brightness

Suggested LEDs are red/green/orange/yellow.

The colour of the LED will have a bearing on how the vactrol works.

So experimentation is in order.

This is an example of a DIY vactrol.

Its part of a Eurorack module:
The NonlinearCircuits Dual LPG

I read somewhere, that yellow LEDs make the most efficient circuit.

The best match is I think between green and green/yellow. A blue LED is probably the worst possible choice.

Also remember that LED colours are normally not standard or very precise from one manufacturer to another.

Here is another example of a DIY vactrol.

The NLC Dispersion Delay.

I used black shrink wrap.

To get the best possible performance, the LED and LDR should have equal wavelengths.

This will require checking datasheets to find a good match.

Model	-	Size (mm)	Light resistance(ohm)	Darkness resistance(ohm)	Max voltage (DC):	Max power consumption	Operating temperature
GL5506	-	5X2	2K~5K	0.2M	150V	100mw	-30C° ~+ 70C°
GL5516	-	5X2	5K~10K	0.5M	150V	100mw	-30C° ~+ 70C°
GL5528	-	5X2	10K~20K	1M	150V	100mw	-30C° ~+ 70C°
GL5537-1	-	5X2	20K~30K	2M	150V	100mw	-30C° ~+ 70C°
GL5537-2	-	5X2	30K~50K	2M	150V	100mw	-30C° ~+ 70C°
GL5539	-	5X2	50K~100K	5M	150V	100mw	-30C° ~+ 70C°
GL5549	-	5X2	100K~200K	10M	150V	100mw	-30C° ~+ 70C°