Vactrols are made from a Light Dependent Resistor and an LED incased in opaque material. Applying a voltage to the LED causes current to pass through it and light to be emitted, which falls on the light dependent resistor. If a voltage is applied to the light dependent resistor, it will develop a current which is proportional to the voltage applied to the LED. This makes the light dependent resistor, in effect, a voltage controlled resistor. It’s commonly used in analog synthesiser designs, in particular, modular synthesisers. This is because through the use of a vactrol, almost any parameter which is normally controlled by a potentiometer or variable resistor can become a voltage-controlled parameter. This is the simplest way to introduce voltage control to a circuit.

I experimented with controlling the pitch of an oscillator I created on the breadboard below with an LFO I also created on the same breadboard. There’s test audio below the picture.

There are a couple things I need to change in future revisions of this circuit. First off I’m using a square wave LFO modulating the pitch creating a siren like sound. On one hand, if I’m still planning to design a circuit based off ultrasonic repellers this would be useful in producing similar reactions to the circuit bent ultrasonic repeller. On the other hand, my ideas for the installation are evolving away from creating alarming sounds and ultrasonic repellers. Instead, moving towards something visually similar to Eirik Brandal’s work pictured below and similar in terms of visitor interaction to the works of Peter Vogel. To fit with these new parameters I should create a smoother waveform(triangle or sine) which means creating a buffer.