88 Nicolas Collins
The basic concept of the blinking LED chopping audio can be extended from
simple oscillators to more complicated control circuits. You can connect the
various outputs of the Divider circuit shown in Chapter 21 to multiple
LED/photocell gates to chop multiple sound sources in rhythmic patterns --
Hacking Dub! Or drive the LED with the output of the cascaded gated
oscillators we made in Chapter 20 -- crazy rhythms! Just don’t forget to include a
resistor between each divider output and its LED.
As with the flashlight-in-the-closet experiment, these circuits do not produce a
total mute when off -- some of your audio signal will continue to bleed through
even when the LED is off. The amount of bleed will depend largely on the choice
of photocell and how effectively they are shielded from outside light. The
photocell should have as large a difference as possible between “on” and “off”
resistance, but is best picked by ear, by substituting different choices into the
circuit.
If the masses of electrical tape offend your sensibilities, you can put the photocell
and LED inside an opaque soda straw, or the plastic sleeve of a mini plug or
guitar plug – you may want to put some BluTak or opaque silicon sealant into
the ends of the tubes to prevent light leakage -- once again, be careful to avoid
shorting the leads against one another. “Heat shrink tubing” is another tidy
solution to light isolation: slide narrow pieces around the legs of the LED and
photocell to insulate them from shorting against one another; slide a wider piece
over the LED and cell; nuzzle the two components tightly together; apply heat
from a hair-dryer to shrink the tubing tight around them. Voilá! A
microelectronic “Bruit Secret.”
This type of optical gating is much prized by audiophiles for its sonic purity.
Only a few small, if confusing, additions stand between these simple circuits and
some very expensive studio noise gates, compressors and limiters.
Other uses for Photocells
As you should grasp by now, the photocell is resistor like any other, but for its
Nosferatu-like response to light (it shrinks from it.) You can substitute a
photocell for most resistors and pots, and then modulate that resistance with a
light -- either performed (flashlights, shadows, etc.) or automated (the blinking
LEDs.) If you already have a toy or oscillator whose pitch is controlled by a
photocell, press a blinking LED against the cell and hear what happens (similar
to the toy cross-modulations we tried at the end of Chapter 15) -- controlling the
pitch of an audio frequency oscillator with a blinking LED of a slower oscillator
running at a suitable B.P.M yields a pleasingly Disco-tinged “syn-drum” swoop.
Sometimes, if its “on” resistance is low enough, a photocell can be substituted for
a low-current switch. If your toy has switches to trigger sounds or enable
functions, try paralleling a switch with the photocell: connect the two photocell
legs to the points on the toy’s circuit board that are joined when the switch is
closed. Run the LED at a slow tempo and see if the chosen event is triggered