An electronic component sometimes used in analog synthesizer designs, and particular in certain modular synthesizer applications. A vactrol consists of two components incorporated into one package: a light-emitting diode (LED), and a photoresistor (a resistor whose resistance drops when it is exposed to light). Applying a voltage to the LED causes current to pass through it and light to be emitted, which falls on the photoresistor. If a voltage is applied to the photoresistor, it will develop a current which is proportional to the voltage applied to the LED. This makes the photoresistor, in effect, a voltage-controlled resistor.

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, by wiring in the vactrol's photoresistor in series with, or in place of, the potentiometer. Applying a voltage to the vactrol's LED has the same effect as turning up the knob on the potentiometer. However, vactrols have certain difficulties: the LED, like all diodes, has a minimum cutoff voltage that must be exceeded before it will begin to light up, and the photoresistor is often not very linear in its response. Further, photoresistor response is also usually not symmetrical; it will rapidly decrease resistance in response to increasing light from the LED, but the resistance may build back up rather slowly when the LED is cut off. All of this makes accurate scaling of the circuit response to control voltage rather difficult, and the circuit designer must carefully choose from among the available vactrol response curves to get the desired response. Despite all this, vactrols are heavily used particularly in the modular and DIY communities, because they are simple to incorporate into a circuit and relatively damage-resistant. And many uses have been found for the non-linearities; for example, the photoresistor's saturation properties have been used to implement a number of audio compressor circuits.

Currently the sole supplier of vactrols is Perkin-Elmer, which markets about ten different types with different response characteristics. Physically, the devices are rather odd-looking, with the LED and photoresistor incorporated in a cylindrical plastic casing with leads sticking out of both ends. The LED leads are at one end and the photoresistor leads are at the other end.

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