Pairing a blinking LED with a photocell inside opaque tubing creates an optically isolated audio gate, panner, or ring modulator

Wrapping a blinking LED and a photocell together (insulated from each other and from external light) creates a voltage-controlled audio gate: when the LED is on, the photocell’s resistance drops and audio passes; when off, it rises and audio is attenuated. Varying the blink rate produces effects from rhythmic gating (slow BPM) through ring-modulator-like distortion (audio frequencies). Two out-of-phase LEDs driving two photocells create an automatic panner or crossfader. Using divider chip outputs to drive multiple LED/photocell pairs in rhythmic patterns creates multi-channel rhythmic gating. The technique is historically connected to David Behrman, Larry Austin, and Voice Crack’s light-controlled electronics.

Examples

Wrap a 74C14 oscillator’s output LED against a photocell with heat-shrink tubing; connect photocell in series with an audio signal; slow oscillator creates a tremolo; fast oscillator creates a buzzy ring-mod texture.

Assessment

Explain why the LED must be shielded from external light in this circuit, and describe what happens to the audio gate behaviour as the oscillator controlling the LED is tuned from 1 Hz up to 1 kHz.