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.