What does a photocell do in a pedal?

[phyrexia]

I am seeing a lot of pedals with them but I don't know what they do. Any help?

[fusionid]

looks pretty

[dot-dot-dot]

Basically it lets you turn a voltage into a resistance. One side is a light bulb (or LED), the other is a light-dependant resistor (or sometimes a phototransistor).

 

The upshot is that you apply a voltage to the first side (such as the output of an oscillator circuit) and you get a varying voltage on the other side, which can then be used in a filter circuit, or any other circuit you want.

 

Usually a varying resistance is more "usable" in a circuit than a varying voltage is.

[zachary vex]

some pedals have a photocell mounted on a 1/4" plug attached to an expression input... this allows you to change some aspect of the sound using available ambient light and your hand or foot creating a shadow on the cell.

 

inside a pedal, a photocell (a true one) is a near-perfect resistive device dependent on light to set it's value. it can make many circuits sound very pure yet allow modulation of some sort... for example, in the world of univibe clones, you'll find some units use a light bulb driving 4 photocells in an equally-spaced arrangement around the bulb. this gives the univibe a very special sort of quality... the near-perfect resistor simulation of a photocell can keep unwanted "rough" distortion tones out of the signal path. on the other hand, fets can be used which introduce slight distortions or create the need for excess circuitry that distracts from the tone... they allow control without the use of a light bulb (which draws a lot of power and usually requires AC) but have a different audio result.

 

some univibe clones use LEDs (easily driven by batteries) to illuminate the photocells... they react so quickly to current change and have such a different response curve that the resulting sound is not much like a univibe, unless great care is taken in the drive circuitry to simulate the lightbulb response.