Two photoresistors in opposing positions create a passive light-controlled stereo panner requiring no power

Extending the single-photoresistor optical gate, two photoresistors can route a single audio source between two amplifier outputs depending on light direction. In the basic panner: audio signal connects through two photoresistors to two separate output jacks. When light hits the first cell, its resistance drops, passing signal to output 1; the second cell in darkness resists and blocks output 2. Moving the light source pans between left and right. For a crossfader between two sources: two photoresistors facing opposite directions, two sources feeding them respectively. Using a blinking LED driven by an oscillator creates auto-pan tremolo at the LED’s oscillation rate. Using a fan blade between LED and cells creates rhythmic stereo tremolo.

Examples

Place two photoresistors side-by-side facing a moving light source; left cell faces away from center, right cell faces toward center. Sweep light left: energy goes left; sweep right: energy goes right. Auto-pan: drive the LED from a 74C14 oscillator.

Assessment

How is a light-controlled crossfader different from a panner? What must be true about the light source and cell positions for the panner to achieve full stereo separation?