Photocell

A light-dependent resistor (LDR), also called a photoresistor, is a light controlled variable resistor. This means that the resistance of the resistor decreases as the light intensity increases. Photocells are popular in dynamics compressors to control gain reduction, but they can also be found in some guitar amp and stompbox tremolo circuits, with the oscillating light controlling signal level. Since there is an innate latency (delay) between light exposure and the decrease in resistance, usually around 10ms, light-dependent resistors are used more for “smoothing” type of compression, where clamping down instantly on the initial attack isn’t desired.