VACTROL BASICS

ANALOG ELEKTRIC ELECTRO-MUSIC BLOG

I HAVE BEEN INTERESTED IN ELECTRONICS SINCE AROUND THE TIME I COULD WALK AND TALK!
MY PARTICULAR AREA OF INTEREST IS AUDIO ELECTRONICS, RELATING TO MUSIC PRODUCTION AND CREATION.
I REGULARLY EXPERIMENT WITH SUCH CIRCUITS AND THIS BLOG IS DEDICATED TO DISCUSSION REGARDING THESE EXPERIMENTS.


My recent experiments have been concerned with the use of vactrols in the application of sound creating and modifying circuits. 


For those unfamiliar with vactrols, they are a form of analogue opto-coupler, with an LED (usually, filament lamps and neon bulbs have featured in some vintage designs) shining on a light dependent resistor (LDR).


They can be used in any electronic music system to replace a resistor, permitting the resistor to be altered using a control voltage or current. 

ILLUSTRATION OF A VACTROLS OPERATION:

The LED brightness is largely directly proportional to the LED current.

The LED voltage quoted as Vf on datasheets is around 1.5 volts typically.

VACTROLS TEND TO BE QUITE PRICEY AND DIFFICULT TO OBTAIN OUTWITH THE US, HOWEVER IT'S EASY TO 'GROW YOUR OWN':

A SIMPLE APPLICATION- A VOLTAGE CONTROLLED AMPLIFIER:

A SIMPLE VCA CAN BE MADE USING A POTENTIAL DIVIDER WITH THE VACTROL LDR IN ONE ARM

There are a number of practical shortcomings with this circuit, however it illustrates in a simple way how they can be used in an electronic music application. 

HOW IT WORKS:

The input signal enters on the left side via the signal input and ground(GND)

connections. The control voltage is applied between the control voltage (CV) input and GND. When the control voltage is below the LED voltage (approx 1.5 volts) the LED is completely dark and the LDR is in complete darkness and has a very high resistance, like 50 Megohms (50 M =50 million ohms) the resistor to ground on the other hand is much lower and the voltage at the junction is extremely low being equal to:

                               Vout= Vin x (1k/1k+50 m)

 i.e about 50 000ths of the input signal or -63dB (decibels) if a greater degree of attenuation (signal reduction) is required the fixed resistor could be reduced to a lower value, e.g. a 100 ohm resistor would yield an output 500 000 times lower than the input ( about -83 dB)

 At the other end of the scale when the maximum current is passed through the LED the bright light shining on the LDR causes it to have a low resistance, of around 2=300 ohms, the output signal will then be almost as large as the input since:

                Vout=Vin x (1k/1k+200)

which yields a value of 5 sixths of the input level ( around -1.6dB ) Almost the same level as the input.

Practical difficulties are that the input and output impedance vary with the setting of the control voltage / led current. Also that the control input requires a lot of current to light the led and therefore may overload any controlling circuit. 

All of these shortcomings can be overcome using op-amps to buffer the signals.

The inputs now have fixed high input impedances, presenting controlling circuits with minimal demand. The Output on the other hand now has a fixed low impedance and is less likely to be overloaded by subsequent circuits in the signal chain.

NEXT: A MORE COMPREHENSIVE LED DRIVER